BCM270X_DT: Make mmc overlay compatible with current firmware

The original DT overlay logic followed a merge-then-patch procedure,
i.e. parameters are applied to the loaded overlay before the overlay
is merged into the base DTB. This sequence has been changed to
patch-then-merge, in order to support parameterised node names, and
to protect against bad overlays. As a result, overrides (parameters)
must only target labels in the overlay, but the overlay can obviously target nodes in the base DTB.

mmc-overlay.dts (that switches back to the original mmc sdcard
driver) is the only overlay violating that rule, and this patch
fixes it.

arch/arm/boot/dts/overlays/mmc-overlay.dts

@@ -7,13 +7,13 @@
 	fragment@0 {
 		target = <&mmc>;
 
-		__overlay__ {
+		frag0: __overlay__ {
 			brcm,overclock-50 = <0>;
 		};
 	};
 
 	__overrides__ {
-		overclock_50     = <&mmc>,"brcm,overclock-50:0";
-		force_pio        = <&mmc>,"brcm,force-pio?";
+		overclock_50     = <&frag0>,"brcm,overclock-50:0";
+		force_pio        = <&frag0>,"brcm,force-pio?";
 	};
 };