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>

board/freescale/mx6qarm2/mx6qarm2.c

@@ -110,6 +110,11 @@ struct fsl_esdhc_cfg usdhc_cfg[2] = {
 	{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;

board/freescale/mx6sabresd/mx6sabresd.c

@@ -235,6 +235,11 @@ struct fsl_esdhc_cfg usdhc_cfg[3] = {
 #define USDHC2_CD_GPIO	IMX_GPIO_NR(2, 2)
 #define USDHC3_CD_GPIO	IMX_GPIO_NR(2, 0)
 
+int board_mmc_get_env_dev(int devno)
+{
+	return devno - 1;
+}
+
 int board_mmc_getcd(struct mmc *mmc)
 {
 	struct fsl_esdhc_cfg *cfg = (struct fsl_esdhc_cfg *)mmc->priv;

board/freescale/mx6slevk/mx6slevk.c

@@ -170,6 +170,11 @@ static struct fsl_esdhc_cfg usdhc_cfg[3] = {
 	{USDHC3_BASE_ADDR, 0, 4},
 };
 
+int board_mmc_get_env_dev(int devno)
+{
+	return devno;
+}
+
 int board_mmc_getcd(struct mmc *mmc)
 {
 	struct fsl_esdhc_cfg *cfg = (struct fsl_esdhc_cfg *)mmc->priv;

board/freescale/mx6sxsabresd/mx6sxsabresd.c

@@ -316,6 +316,11 @@ static struct fsl_esdhc_cfg usdhc_cfg[3] = {
 #define USDHC3_PWR_GPIO	IMX_GPIO_NR(2, 11)
 #define USDHC4_CD_GPIO	IMX_GPIO_NR(6, 21)
 
+int board_mmc_get_env_dev(int devno)
+{
+	return devno - 1;
+}
+
 int board_mmc_getcd(struct mmc *mmc)
 {
 	struct fsl_esdhc_cfg *cfg = (struct fsl_esdhc_cfg *)mmc->priv;