ARM: at91: at91sam9260: add missing pinctrl-names on mmc

pinctrl-names was missing causing mmc pinctrl to never be requested.
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
[nicolas.ferre@atmel.com: added a commit message taken from Ludovic]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/boot/dts/at91sam9260.dtsi

@@ -572,6 +572,7 @@ mmc0: mmc@fffa8000 {
 				interrupts = <9 IRQ_TYPE_LEVEL_HIGH 0>;
 				#address-cells = <1>;
 				#size-cells = <0>;
+				pinctrl-names = "default";
 				status = "disabled";
 			};