usb: dwc3: of-simple: add support for GDSC regulator

On QCOM platforms, the GDSC regulator is required to be enabled
prior to enabling the clocks. Add support for this optional
regulator.

Change-Id: Ic9484af727e8a7176521e9366e38d852c7747995
Signed-off-by: Jack Pham <jackp@codeaurora.org>