usb: dwc3: Use devm_regulator_get_optional() for dpdm regulator

dpdm regulator is only required for primary usb to make sure
dp and dm lines are in high-z state prior to performing charger
detection. Calling devm_regulator_get() for secondary usb returns
handle for dummy regulator as dpdm property is not present. As a
result secondary dwc3 does not enter low power mode upon boot up.
Fix this by calling devm_regulator_get_optional() which returns
error if regulator is not found.

Change-Id: I387560cf15f3f5505caa1c6418b6e17a4ab14c3b
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>