ARM: dts: imx6dl-yapp4: Use aliases to set custom MMC device indexes

Until commit fa2d0aa9 ("mmc: core: Allow setting slot index via
device tree alias") was introduced, our usdhc3 and usdhc4 devices
were enumerated as mmc0 and mmc1. The mmc1 device is used to boot/update
the board and its name must be fixed.

With the referenced commit, aliases from imx6qdl.dtsi took effect.
Override the aliases to get back the original device indexes.
Signed-off-by: Michal Vokáč <michal.vokac@ysoft.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6dl-yapp4-common.dtsi

@@ -11,6 +11,8 @@ / {
 	aliases: aliases {
 		ethernet1 = &eth1;
 		ethernet2 = &eth2;
+		mmc0 = &usdhc3;
+		mmc1 = &usdhc4;
 	};
 
 	backlight: backlight {