ARM: dts: at91: sam9g20ek: enable udc vbus gpio pinctrl

We set the PIOC to GPIO mode. This way the pin becomes an
input signal will be usable by the controller. Without
this change the udc on the 9g20ek does not work.

Cc: nicolas.ferre@microchip.com
Cc: ludovic.desroches@microchip.com
Cc: alexandre.belloni@bootlin.com
Cc: linux-arm-kernel@lists.infradead.org
Cc: kernel@pengutronix.de
Fixes: 5cb4e735 ("ARM: at91: add at91sam9g20ek boards dt support")
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20221114185923.1023249-3-m.grzeschik@pengutronix.de