scsi: ufs: obtain VCCQ2 voltage level from DT prior to hard code

Some Vendor has their specific VCCQ2 voltage range which is different
with universal UFS spec. So we need to read VCCQ2 voltage level from
DT prior to from hard code.

Change-Id: I08a01fded6e2431c76110fe5b9e1e71f86177a63
Signed-off-by: Ziqi Chen <ziqichen@codeaurora.org>