drivers: mfd: Mark CPR bank registers as volatile

Mark CPR bank registers as volatile so that
writes to these registers get updated correctly
in hardware.

CRs-Fixed: 1081673
Change-Id: I92d0511b4e9912dfa346378784d811f6606df205
Signed-off-by: Phani Kumar Uppalapati <phaniu@codeaurora.org>