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/platinum_picon_defconfig b/configs/platinum_picon_defconfig
index 54d88b2..c45ce14 100644
--- a/configs/platinum_picon_defconfig
+++ b/configs/platinum_picon_defconfig
@@ -23,7 +23,6 @@
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_I2C=y
 CONFIG_CMD_MMC=y
-CONFIG_CMD_NAND=y
 CONFIG_CMD_NAND_TRIMFFS=y
 CONFIG_CMD_USB=y
 # CONFIG_CMD_SETEXPR is not set
@@ -41,6 +40,8 @@
 CONFIG_MTDPARTS_DEFAULT="mtdparts=gpmi-nand:14M(spl),2M(uboot),512k(env1),512k(env2),495M(ubi0),14M(res0),2M(res1),512k(res2),512k(res3),-(ubi1)"
 CONFIG_CMD_UBI=y
 CONFIG_ENV_IS_IN_NAND=y
+CONFIG_NAND=y
+CONFIG_NAND_MXS=y
 CONFIG_PHYLIB=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y