regmap: Skip read-only registers in regcache_sync()

regcache_sync() spews warnings when a value was cached for a read-only
register as it tries to write all registers no matter whether they are
writable or not.  This patch adds regmap_wrtieable() checks for
avoiding it in regcache_sync_block_single() and regcache_block_raw().

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mark Brown <broonie@kernel.org>