phy: qcom-ufs: don't error out if vddp-ref-clk-supply is absent

vddp-ref-clk-supply is controlled by UFS host, so it can be absent in UFS
PHY's DT and shouldn't be considered as error.

Change-Id: I7f8af258c59851ccd8adceb73f9a6da62472e915
Signed-off-by: Can Guo <cang@codeaurora.org>