mtd: ensure CMD_NAND is compiled when its options are selected

In some files, options of CMD_NAND are selected but not the command
itself. Fix this inconsistency.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
diff --git a/configs/mx28evk_spi_defconfig b/configs/mx28evk_spi_defconfig
index f09777c..3adcc75 100644
--- a/configs/mx28evk_spi_defconfig
+++ b/configs/mx28evk_spi_defconfig
@@ -21,6 +21,7 @@
 # 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_USB=y
 CONFIG_CMD_DHCP=y