scsi: ufs-mediatek: Support VA09 regulator operations

Some MediaTek UFS platforms need to control VA09 power
specifically. Provide such control according to the device tree binding.

Link: https://lore.kernel.org/r/20201029115750.24391-3-stanley.chu@mediatek.com


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