The use of GPIOs should be optional for the generic phy, otherwise
the Altera SOCFPGA platform at least is broken.

Fixes breakage caused by a combination of e9f2cefb "usb: phy:
generic: migrate to gpio_desc" and 135b3c43 "usb: dwc2: platform:
add generic PHY framework support".
Reviewed-by: NRobert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: NPaul Zimmerman <paulz@synopsys.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>