gpio: Fix GPIO high-speed state inconsistency

There are two valid ways to prepare a high-speed GPIO output:

```rust
pub fn configure_led(pad: LedPadType) -> LED {
    let mut led = hal::gpio::GPIO::new(pad);
    led.set_fast(true);
    led.output()
}
```

and

```rust
pub fn configure_led(pad: LedPadType) -> LED {
    let mut led = hal::gpio::GPIO::new(pad).output();
    led.set_fast(true);
    led
}
```

the former will put the GPIO into high-speed, or 'fast,' mode before
setting 'output mode.' The latter will put the GPIO into output mode
before fast mode.

After transitioning into fast mode, the GPIO will start to reference
a different GPIO register block. The issue is that, after entering
fast mode, the output / input state of the pin is not maintained. In
the second snippet, the set_fast(true) call ends up reverting the
GPIO state back to 'input,' since the newly-referenced register block
does not maintain the same GPIO input / output configuration.

This commit updates the set_fast() method to maintain the GPIO I/O
state for the new register block. It makes it so that either of the
above patterns work.

imxrt-hal/src/gpio.rs

@@ -110,20 +110,54 @@ where
     pub fn set_fast(&mut self, fast: bool) -> bool {
         self.gpr()
             .map(|gpr| {
-                cortex_m::interrupt::free(|_| unsafe {
+                cortex_m::interrupt::free(|cs| unsafe {
                     let v = core::ptr::read_volatile(gpr);
 
+                    let was_output = self.is_output();
+
                     if fast {
                         core::ptr::write_volatile(gpr, v | self.offset());
                     } else {
                         core::ptr::write_volatile(gpr, v & !self.offset());
                     }
 
+                    // At this point, calls to set_output / set_input will refer to the 'fast'
+                    // or 'slow' GPIO register block.
+                    if was_output {
+                        self.set_output(cs);
+                    } else {
+                        self.set_input(cs);
+                    }
+
                     true
                 })
             })
             .unwrap_or(false)
     }
+
+    /// Returns `true` if the pin is configured as an output pin
+    fn is_output(&self) -> bool {
+        // Safety: atomic read
+        unsafe { ral::read_reg!(ral::gpio, self.register_block(), GDIR) & self.offset() != 0 }
+    }
+
+    /// Configure the GPIO as an output
+    fn set_output(&self, _: &cortex_m::interrupt::CriticalSection) {
+        // Safety: critical section, enforced by API, ensures consistency
+        unsafe {
+            ral::modify_reg!(ral::gpio, self.register_block(), GDIR, |gdir| gdir
+                | self.offset());
+        }
+    }
+
+    /// Configure the GPIO as an input
+    fn set_input(&self, _: &cortex_m::interrupt::CriticalSection) {
+        // Safety: critical section, enforced by API, ensures consistency
+        unsafe {
+            ral::modify_reg!(ral::gpio, self.register_block(), GDIR, |gdir| gdir
+                & !self.offset());
+        }
+    }
 }
 
 impl<P> GPIO<P, Input>
@@ -143,11 +177,7 @@ where
 
     /// Set the GPIO as an output
     pub fn output(self) -> GPIO<P, Output> {
-        // Safety: critical section ensures consistency
-        cortex_m::interrupt::free(|_| unsafe {
-            ral::modify_reg!(ral::gpio, self.register_block(), GDIR, |gdir| gdir
-                | self.offset());
-        });
+        cortex_m::interrupt::free(|cs| self.set_output(cs));
         GPIO {
             pin: self.pin,
             dir: PhantomData,
@@ -167,11 +197,7 @@ where
 {
     /// Transition the pin back to an input
     pub fn input(self) -> GPIO<P, Input> {
-        // Safety: critical section ensures consistency
-        cortex_m::interrupt::free(|_| unsafe {
-            ral::modify_reg!(ral::gpio, self.register_block(), GDIR, |gdir| gdir
-                & !self.offset());
-        });
+        cortex_m::interrupt::free(|cs| self.set_input(cs));
         GPIO {
             pin: self.pin,
             dir: PhantomData,