usb: phy: make GPIOs optional for the generic phy

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: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>