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/xilinx_zynqmp_mini_nand_single_defconfig b/configs/xilinx_zynqmp_mini_nand_single_defconfig
index ca53edf..1f518a7 100644
--- a/configs/xilinx_zynqmp_mini_nand_single_defconfig
+++ b/configs/xilinx_zynqmp_mini_nand_single_defconfig
@@ -47,6 +47,7 @@
 # CONFIG_DM_WARN is not set
 # CONFIG_DM_DEVICE_REMOVE is not set
 # CONFIG_MMC is not set
+CONFIG_MTD=y
 CONFIG_MTD_RAW_NAND=y
 CONFIG_NAND_ARASAN=y
 # CONFIG_EFI_LOADER is not set