usb: phy: Make ref_clk optional for driver probe success

Ref_clk is needed only in case of differential clock. There
are cases in which ref_clk is coming directy from CXO pad
and connected to a differential pin. In this case ref_clk
will not be passed from DT node and driver must success its
probe. So this change makes ref_clk optional for successful
completion of probe.

Change-Id: I80a293e381b69306eeca916da36edba06a3879b6
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>