ARM: dts: msm: Add register mask for bits to set in FD on 8996

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: If922771631293615caa7e910e94a89c087e54d04
Signed-off-by: Sunid Wilson <sunidw@codeaurora.org>