ASoC: soc-cache: Introduce the cache_bypass option

This is primarily needed to avoid writing back to the cache
whenever we are syncing the cache with the hardware.  This gives a
performance benefit especially for large register maps.

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