scsi: ufs: provide sysfs attribute to select the PM level

This patch provides the sysfs attribute to choose the power management
level for UFS runtime and system suspend.

Change-Id: I5beedadff1ac3a1ac1f8d36ab88fc3b2ecb75e88
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>