scsi: ufs: add option to change default UFS power management level

UFS device and link can be put in multiple different low power modes hence
UFS driver supports multiple different low power modes. By default UFS
driver selects the default (optimal) low power mode (which gives moderate
power savings and have relatively less enter and exit latencies) but
we might have to tune this default power mode for different chipset
platforms to meet the low power requirements/goals. Hence this patch
adds option to change default UFS low power mode (level).

Change-Id: I45aaae9f46beb3b5d38bcc6dcbd728e79677276c
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>