UPSTREAM: scsi: ufs-mediatek: Introduce low-power mode for device power supply

Allow device power supply to enter low-power mode.

Link: https://lore.kernel.org/r/20200601104646.15436-4-stanley.chu@mediatek.com


Reviewed-by: Pengshun Zhao <pengshun.zhao@mediatek.com>
Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Bug: 151050916
(cherry picked from commit 488edafb1120f715bebd80b6fe4089f83fb082b2)
Change-Id: Ibe3cd1f11305656f7c597940b75e0c6d71c4a802
Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>