Add dbg_register_dump variant vendor implementation in MediaTek UFS driver.

Link: https://lore.kernel.org/r/20200117035108.19699-2-stanley.chu@mediatek.comReviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>