ASoC: sta32x: Convert to regmap

Long term all drivers should be using regmap directly. This is more
idiomatic and moves us towards the removal of the ASoC level cache
code.

The initialiasation of reserved register bits in probe() is slightly odd
as the defaults being written don't appear to match the silicon defaults
but the new code should have the same effect as the old code.

The watchdog code will now unconditionally do a mute and unmute when
resyncing but since we only sync when we are very sure there is something
to sync this should have no impact.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Johannes Stezenbach <js@sig21.net>