scsi: ufs-mediatek: Allow unbound mphy

Allow unbound MPHY module since not every MediaTek UFS platform needs
specific MPHY control.

Link: https://lore.kernel.org/r/20200601104646.15436-6-stanley.chu@mediatek.com


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