scsi: ufs: Get UFS ref_clk min/max voltage from device tree

This change allows selecting the UFS ref clk voltage level
by setting ref-clk-min-uV and ref-clk-max-uV in device tree.
If ref_clk voltage level setting is not found in device tree,
use default values provided by driver.

Change-Id: Ia3c0c61a508045448057cc3057a30546a458bfc9
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>