wandboard: Add mmc0 alias

Add a mmc0 alias so that U-Boot proper can associate mmc0
with the boot SD card.

Signed-off-by: Fabio Estevam <festevam@gmail.com>

arch/arm/dts/imx6qdl-wandboard.dtsi

@@ -8,6 +8,10 @@
 #include <dt-bindings/gpio/gpio.h>
 
 / {
+	aliases {
+		mmc0 = &usdhc3;
+	};
+
 	chosen {
 		stdout-path = &uart1;
 	};