ARM: dts: msm: Remove unused ref_clk from USB

As of commit b44b9c5a ("usb: dwc3-msm: Remove voting for
ref_clk") the USB device no longer references the ref_clk. Remove
the unused entries from msm8996, mdm9640 and mdmcalifornium.

Change-Id: I3979e56293c8729cf89b32677bb23216ce68f120
Signed-off-by: Jack Pham <jackp@codeaurora.org>