ARM: dts: at91: sama5d2_ptc_ek: fix nand pinctrl

The drive strength has to be set to medium otherwise some data
corruption may happen.

Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>