ASoC: Implement runtime PM for WM8994/58

This allows us to communicate our power management state back to the
parent device, allowing it to do a full power down when the device is
idle.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>