ASoC: wm_adsp: Put DSP into low power state between loading and running

Between when we load the DSP and when it actually starts running put the
core into a lower power state where the memory is retained but nothing
is clocked.

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