scsi: ufs: add UFS power management support

This patch adds support for UFS device and UniPro link power management
during runtime/system PM.

Main idea is to define multiple UFS low power levels based on UFS device
and UFS link power states. This would allow any specific platform or pci
driver to choose the best suited low power level during runtime and
system suspend based on their power goals.

Change-Id: Ifb89253fc9ebb3c512cc6d8d70a7055dc1c78fba
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>