phy: qcom-ufs: Update documentation for ref_clk

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: I13c4bf671716d5ad5bc4100a2155a77d541181c3
Signed-off-by: Can Guo <cang@codeaurora.org>