usb: dwc3-msm: Make phy_com_reset optional for power collapse

Some targets do not have the phy_com_reset clock, so allow it
to be optional in order to still support GDSC power collapse.

Change-Id: I1438a5fc42c6307e904e6c639ff35bad2f69c335
Signed-off-by: Jack Pham <jackp@codeaurora.org>