clk: tegra: Add closed loop support for the DFLL

With closed loop support, the clock rate of the DFLL can be adjusted.

The oscillator itself in the DFLL is a free-running oscillator whose
rate is directly determined the supply voltage. However, the DFLL
module contains logic to compare the DFLL output rate to a fixed
reference clock (51 MHz) and make a decision to either lower or raise
the DFLL supply voltage. The DFLL module can then autonomously change
the supply voltage by communicating with an off-chip PMIC via either I2C
or PWM signals. This driver currently supports only I2C.

Signed-off-by: Tuomas Tynkkynen <[email protected]>
Signed-off-by: Mikko Perttunen <[email protected]>
Acked-by: Peter De Schrijver <[email protected]>
Acked-by: Michael Turquette <[email protected]>
Signed-off-by: Thierry Reding <[email protected]>