imx: mx6: implement board_mmc_get_env_dev

Implement board_mmc_get_env_dev for the boards.

Following is examples:
SD1/SD2/SD3: return devno;
SD2/SD3: return devno - 1;
SD2/SD4: if (devno == 2), return dev - 2; return dev - 1;

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
diff --git a/board/freescale/mx6qarm2/mx6qarm2.c b/board/freescale/mx6qarm2/mx6qarm2.c
index 98ccdb7..5aae721 100644
--- a/board/freescale/mx6qarm2/mx6qarm2.c
+++ b/board/freescale/mx6qarm2/mx6qarm2.c
@@ -110,6 +110,11 @@
 	{USDHC4_BASE_ADDR},
 };
 
+int board_mmc_get_env_dev(int devno)
+{
+	return devno - 2;
+}
+
 int board_mmc_getcd(struct mmc *mmc)
 {
 	struct fsl_esdhc_cfg *cfg = (struct fsl_esdhc_cfg *)mmc->priv;