usb: gadget: pxa27x_udc device-tree documentation

Add documentation for device-tree binding of arm PXA 27x udc (usb
device) driver.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: devicetree@vger.kernel.org
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>