ARM: dts: at91: sama5d4: fix pinctrl compatible string

The compatible string is incorrect. Add atmel,sama5d3-pinctrl since
it's the appropriate compatible string. Remove the
atmel,at91rm9200-pinctrl compatible string, this fallback is
useless, there are too many changes.

Signed-off-by: Santiago Esteban <Santiago.Esteban@microchip.com>
Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Cc: stable@vger.kernel.org #v3.18
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>