driver: mfd: Mark RCO calibration registers are volatile

RCO calibration codec registers are read only and they can
be modified at any time during codec operation. Mark them as
volatile so that reads to these registers always happen from
hardware instead of cache.

CRs-Fixed: 974785
Change-Id: I1fb66ca19d0889aa7e21f433f8722af7b527705c
Signed-off-by: Phani Kumar Uppalapati <phaniu@codeaurora.org>