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/bg0900_defconfig b/configs/bg0900_defconfig
index ed0448d..602eb60 100644
--- a/configs/bg0900_defconfig
+++ b/configs/bg0900_defconfig
@@ -17,7 +17,6 @@
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_GPIO=y
-CONFIG_CMD_NAND=y
 CONFIG_CMD_NAND_TRIMFFS=y
 CONFIG_CMD_SF=y
 CONFIG_CMD_SPI=y
@@ -27,6 +26,8 @@
 CONFIG_CMD_CACHE=y
 CONFIG_DOS_PARTITION=y
 # CONFIG_MMC is not set
+CONFIG_NAND=y
+CONFIG_NAND_MXS=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_BAR=y
 CONFIG_SPI_FLASH_STMICRO=y