ARM: dts: zynq: Add mmc alias for zc702/zc706/zed/zybo

Add missing mmc alias.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/arm/boot/dts/zynq-zybo.dts

@@ -13,6 +13,7 @@
 	aliases {
 		ethernet0 = &gem0;
 		serial0 = &uart1;
+		mmc0 = &sdhci0;
 	};
 
 	memory@0 {