scsi: ufs: Read device tree to find UFS's Vccq2 voltage level

This change allows selecting the UFS Vccq2 voltage level by
setting the UFS's entry vccq2-voltage-level in the device tree.
If UFS's Vccq2 voltage level setting is not found in the device
tree,use default values provided by the driver.

Change-Id: Ibe9223e717e63b9edde95b272aaf142e7cf81672
Signed-off-by: Nitin Rawat <nitirawa@codeaurora.org>