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/m28evk_defconfig b/configs/m28evk_defconfig
index 734bc0a..1999fa8 100644
--- a/configs/m28evk_defconfig
+++ b/configs/m28evk_defconfig
@@ -22,7 +22,6 @@
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_MMC=y
-CONFIG_CMD_NAND=y
 CONFIG_CMD_NAND_TRIMFFS=y
 CONFIG_CMD_SF=y
 CONFIG_CMD_SPI=y
@@ -42,6 +41,8 @@
 CONFIG_CMD_UBI=y
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_MMC_MXS=y
+CONFIG_NAND=y
+CONFIG_NAND_MXS=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_USB=y