Convert CONFIG_NAND_MXS to Kconfig

This converts CONFIG_NAND_MXS to Kconfig.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
diff --git a/configs/gwventana_nand_defconfig b/configs/gwventana_nand_defconfig
index be6cf0c..fbfd352 100644
--- a/configs/gwventana_nand_defconfig
+++ b/configs/gwventana_nand_defconfig
@@ -36,7 +36,6 @@
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_I2C=y
 CONFIG_CMD_MMC=y
-CONFIG_CMD_NAND=y
 CONFIG_CMD_NAND_TRIMFFS=y
 CONFIG_CMD_PCI=y
 CONFIG_CMD_USB=y
@@ -57,6 +56,8 @@
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_DM=y
 CONFIG_DWC_AHSATA=y
+CONFIG_NAND=y
+CONFIG_NAND_MXS=y
 CONFIG_PHYLIB=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y