usb: dwc3: qcom: Use GDSC regulator if available

GDSC can be specified as a regulator in device tree as
'USB3_GDSC-supply'. If present, use it and enable it
prior to enabling the clocks.

Change-Id: I50c056dd920b457fe77fd364414657e98413f4d0
Signed-off-by: Jack Pham <jackp@codeaurora.org>