scsi: ufs: Read device tree to find UFS's Vcc voltage level

This change allows selecting the UFS Vcc voltage level by setting the
UFS's entry vcc-voltage-level in the device tree.
If UFS's Vcc voltage level setting is not found in the device tree,
use default values provided by the driver.

Change-Id: Ifc6856d63b672ed6b178f45fd5f139cb2fa56f15
Signed-off-by: Bao D. Nguyen <nguyenb@codeaurora.org>