imx8mp-msc-sm2s: Add mmc aliases

Add mmc alias so that the eMMC is mmc0 and the SD card
is mmc1 to have a well defined device numbering scheme.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Ian Ray <ian.ray@gehealthcare.com>
diff --git a/arch/arm/dts/imx8mp-msc-sm2s-u-boot.dtsi b/arch/arm/dts/imx8mp-msc-sm2s-u-boot.dtsi
index 1a7b530..ce61ca6 100644
--- a/arch/arm/dts/imx8mp-msc-sm2s-u-boot.dtsi
+++ b/arch/arm/dts/imx8mp-msc-sm2s-u-boot.dtsi
@@ -9,6 +9,11 @@
 	model = "MSC SM2S-IMX8MPLUS";
 	compatible = "avnet,sm2s-imx8mp", "fsl,imx8mp";
 
+	aliases {
+		mmc0 = &usdhc3;
+		mmc1 = &usdhc2;
+	};
+
 	wdt-reboot {
 		compatible = "wdt-reboot";
 		wdt = <&wdog1>;