scsi: ufs: ufs-mediatek: Introduce default delay for reference clock

Introduce default delay time for gating or ungating reference clock instead
of ambiguous magic numbers.

The defined value is suitable for all current MediaTek UFS platforms.

Link: https://lore.kernel.org/r/20211016005802.7729-2-stanley.chu@mediatek.com


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