mfd: wcd934x: Mark CPE registers as volatile

Some of the CPE (Codec Processing Engine) registers in wcd934x audio
codec are read-write registers with only few bits that are read-only.
Add change to mark such registers as volatile to avoid reading stale
value from cache for the read-only bits.

CRs-Fixed: 1049012
Change-Id: I6d77cf57e940c9efa6be64565d9ea7d328f0cd9b
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>