ASoC: SSM2602: Model power supply for the digital core as a DAPM widget

Model the power supply for the digital core as a DAPM_SUPPLY widget. This allows
to cleanup the code a bit.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>