scsi: dt-bindings: ufs: Add document for ufs-mediatek

Add UFS and UFS PHY node document for Mediatek SoC chips.

Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>