scsi: ufs: Change power mode at run-time via debug-fs

Add 'power_mode' entry to UFS debug-fs to allow query of current
power mode status and changing the power mode by writing to the
entry a string in the format 'GGLLMM' where:
G - selected gear
L - number of lanes
M - power mode
    (1=fast mode, 2=slow mode, 4=fast-auto mode, 5=slow-auto mode)
First letter is for RX, second is for TX.

Change-Id: Ia48cb2719bb11e66bca923c5f4647a33cbd6c43e
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>