mfd: wcd934x: Mark CPE registers as volatile

Currently only read-only registers are marked as volatile. It is
possible that some registers could be read-write and still have
subset of the bits that are updated by the hardware. Add change
to mark such CPE (codec processing engine) register as volatile.

CRs-Fixed: 1049012
Change-Id: I1cedf646d0df1ab5968df4830502649de305c9d1
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>