regmap: debugfs: Allow writes to cache state settings

Allow the user to write the cache_only and cache_bypass settings.
This can be useful for debugging.

Since this can lead to the hardware getting out-of-sync with the
cache, at least for the period that the cache state is forced, the
kernel is tainted and the action is recorded in the kernel log.

When disabling cache_only through debugfs a cache sync will be performed.

Signed-off-by: Richard Fitzgerald <rf@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>