So far we assumed that the node attributes like amp values remain
during the power state transition of the node itself.  While this is
true for IDT/STAC codecs I've tested, but some other codecs don't seem
behaving in that way.

This patch implements a partial sync mechanism specific to the given
widget node.  Now we've merged the regmap support, and it can be
easily written with regcache_sync_region().
Tested-by: Hui Wang <hui.wang@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>