scsi: ufs: define cpu affinity mask for PM QoS voting

PM QoS request type PM_QOS_REQ_AFFINE_CORES specifies for which CPU
cores the voting is applied to by the cpu affinity mask.
This change defines the cpu mask to be used for the voting in the
device tree node so it can be customized for each target.

Change-Id: I004dea47b42eaf3cdf0489427b2bb894c9982f22
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>