ASoC: mmp-sspa: Add support for the runtime power management

Only turn on the Audio island when it's in use.

This requires keeping track of control register contents instead of
reloading them back from hardware, because they're lost when the power is
off.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20200511210134.1224532-9-lkundrak@v3.sk


Signed-off-by: Mark Brown <broonie@kernel.org>