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>