ASoC: wm_adsp: Replace debugfs lock with more general DSP power lock

Most events around the DSP just need to be locked to ensure that the DSP
can't change power state whilst they are happening. This includes the
debugfs entries and this will make sorting the rest of the locking
simpler.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>