mfd: twl6040: reg_defaults support for regmap

Add reg_defaults to regmap and at the same time implement proper power
state handling with using regcache_cache_only(), regcache_sync() and
regcache_mark_dirty().

This will make sure that we do not need to do restore operations in child
drivers anymore.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>