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.

Reviewed-by: Sujit Reddy Thumma <sthumma@codeaurora.org>
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>