ALSA: hda - Add cache support for COEF read/write

The 16bit COEF read/write is pretty standard for many codecs, and they
can be cached in most cases -- more importantly, they need to be
restored at resume.  For making this easier, add the cache support to
regmap.  If the codec driver wants to cache the COEF access, set
codec->cache_coef flag and issue AC_VERB_GET_PROC_COEF with the coef
index in LSB 8 bits.

Signed-off-by: Takashi Iwai <tiwai@suse.de>