UPSTREAM: 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>

Bug: 151050916
(cherry picked from commit fc4983018fea1237e027a9fc6530a3e04850fbfa)
Change-Id: Ie79d90079976129e37ac99c6b7e2d9b172ebdc73
Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>