scsi: ufs: Allow regulators being always-on

Introduce a flag "always_on" in struct ufs_vreg to allow vendors to keep
the regulator always-on.

Link: https://lore.kernel.org/r/20201207054955.24366-2-stanley.chu@mediatek.com


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