ASoC: littlemill: Use a widget to keep track of AIF2 clocking

Since we only need to clock AIF2 when it's actively in use start up the
FLL for it using a supply widget which supplies AIF2CLK. This both makes
the sequencing more robust and ensures we minimise power consumption.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>