imx: mx6: Change ENV offset to 512K bytes for larger u-boot image

To align with other mx6 boards, change ENV offset from 384KB to
512KB position to fit a larger u-boot image.

Signed-off-by: Ye.Li <B37916@freescale.com>
diff --git a/include/configs/mx6slevk.h b/include/configs/mx6slevk.h
index e6c4130..c567d9d 100644
--- a/include/configs/mx6slevk.h
+++ b/include/configs/mx6slevk.h
@@ -198,7 +198,7 @@
 #define CONFIG_ENV_SPI_MODE             CONFIG_SF_DEFAULT_MODE
 #define CONFIG_ENV_SPI_MAX_HZ           CONFIG_SF_DEFAULT_SPEED
 #else
-#define CONFIG_ENV_OFFSET		(6 * SZ_64K)
+#define CONFIG_ENV_OFFSET		(8 * SZ_64K)
 #define CONFIG_ENV_IS_IN_MMC
 #endif