scsi: ufs-qcom: add sys-fs entries for PM QoS control

Add sys-fs entries to allow user space control PM QoS
latency parameters and enable/disable PM QoS voting.

The entries are added under:
/sys/bus/platform/devices/624000.ufshc/624000.ufshc:ufs_variant/

pm_qos_enable:
write 0 to disable PM QoS, 1 to enable.
Example: "echo 1 > pm_qos_enable"

pm_qos_latency_us:
write the desired value for each cpu group, separated by a comma.
Example: "echo 10,20 > pm_qos_latency_us"

Change-Id: I9797a1e62c4867ab831b4f18cbb1e0ca9834247b
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>
Signed-off-by: Krishna Konda <kkonda@codeaurora.org>
[venkatg@codeaurora.org: resolved trivial merge conflicts]
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>