ASoC: core: Add widget SND_SOC_DAPM_CLOCK_SUPPLY

Adds a supply-widget variant for connection to the clock-framework.
This widget-type corresponds to the variant for regulators.

Signed-off-by: Ola Lilja <ola.o.lilja@stericsson.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>