phy: qcom-ufs: make ref_clk as optional

Per newer UFS PHY design, some platforms may not have the ON/OFF control
for reference clock, hence this clock may be optional.

Change-Id: I26edd35970e88a8d17edded399b63c5d3cc7f5d9
Signed-off-by: Can Guo <cang@codeaurora.org>