board: icore-rqs: Fix mmc get env device

As per USDHC boot eFUSE descriptions:
USDHC3 => devno 2
USDHC4 => devno 3

Linux will detect mmc0, mmc1, mmc2 based on the status
"okay" on usdhc so imx6qdl-icore-rqs.dtsi has enabled
usdhc1, usdhc3 and usdhc4.But U-Boot can detect based
on the aliases so add mmc1, mmc2 for usdhc3 and usdhc4
respectively and return the board_mmc_get_env_dev
by subtracting -1

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>

arch/arm/dts/imx6qdl-icore-rqs.dtsi

@@ -43,6 +43,11 @@
 #include <dt-bindings/clock/imx6qdl-clock.h>
 
 / {
+	aliases {
+		mmc1 = &usdhc3;
+		mmc2 = &usdhc4;
+	};
+
 	memory {
 		reg = <0x10000000 0x80000000>;
 	};

board/engicam/icorem6_rqs/icorem6_rqs.c

@@ -13,7 +13,7 @@ DECLARE_GLOBAL_DATA_PTR;
 #ifdef CONFIG_ENV_IS_IN_MMC
 int board_mmc_get_env_dev(int devno)
 {
-	return devno;
+	return devno - 1;
 }
 #endif