usb: gadget: pxa27x_udc: prepare device-tree support

For this preparation, a preliminary cleanup is done :
  - convert the probing of pxa27x_udc to gpio_desc.
    The conversion is partial because :
     - the platform data still provides a gpio number, not a gpio desc

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