imx: mx6slevk: Add support for USDHC1 and USDHC3 slots

There are three SD/MMC sockets on mx6slevk boards. Implements the
full support for them.
The default boot socket is USDHC2, so the MMC environment is set
to that device.

Signed-off-by: Ye.Li <B37916@freescale.com>
diff --git a/include/configs/mx6slevk.h b/include/configs/mx6slevk.h
index fddedf1..4fcaf51 100644
--- a/include/configs/mx6slevk.h
+++ b/include/configs/mx6slevk.h
@@ -190,7 +190,6 @@
 #define CONFIG_ENV_OFFSET		(6 * SZ_64K)
 #define CONFIG_ENV_SIZE			SZ_8K
 #define CONFIG_ENV_IS_IN_MMC
-#define CONFIG_SYS_MMC_ENV_DEV		0
 
 #define CONFIG_OF_LIBFDT
 #define CONFIG_CMD_BOOTZ
@@ -210,4 +209,9 @@
 #define CONFIG_SF_DEFAULT_MODE		SPI_MODE_0
 #endif
 
+#define CONFIG_SYS_FSL_USDHC_NUM	3
+#if defined(CONFIG_ENV_IS_IN_MMC)
+#define CONFIG_SYS_MMC_ENV_DEV		1	/* SDHC2*/
+#endif
+
 #endif				/* __CONFIG_H */