usb: phy: generic: make vcc regulator optional

phy-generic uses the existance of the property "vcc-supply" to see if a
regulator is optional or not. Use devm_regulator_get_optional() instead
which exists for this purpose. Using devm_regulator_get_optional()
avoids "supply vcc not found, using dummy regulator" messages.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Link: https://lore.kernel.org/r/20221012132754.292151-1-s.hauer@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>