msm: fd: Add register mask to identify the bits to change

Currently we read register address offset and value
from dtsi and only set the bits that are currently
not set in the register. We do not consider the
use case were we would like to unset a register bit.
In order to add this capability we need a register mask
which is used to identify the register bits that we would
like to change.

Change-Id: Ia42613a4d99e31b30de6180f6d22838c682f5313
Signed-off-by: Sunid Wilson <sunidw@codeaurora.org>