scsi: dt-bindings: ufs: Add VCC capability on MediaTek UFS driver

Add VCC supply for ufs-mediatek driver to provide power-saving operation
during low-power modes. For example VCC can be turned-off during system
suspend and turned-on after system is resumed.

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