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