This function has no more non regmap user, which means we can remove the
implementation of the function and associated functions and structure
fields.

For convenience we keep a static inline version of the function that
forwards calls to regcache_sync() unconditionally.
Signed-off-by: NLars-Peter Clausen <lars@metafoo.de>
Signed-off-by: NMark Brown <broonie@kernel.org>