mtd: ensure MTD is compiled when there is a SPI NOR flash using MTD

MTD must be enabled when there is a SPI NOR flash using the
SPI_FLASH_MTD config entry.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
diff --git a/configs/tplink_wdr4300_defconfig b/configs/tplink_wdr4300_defconfig
index 3a518d0..40ca10f 100644
--- a/configs/tplink_wdr4300_defconfig
+++ b/configs/tplink_wdr4300_defconfig
@@ -25,6 +25,7 @@
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_CLK=y
 CONFIG_DM_SPI_FLASH=y
+CONFIG_MTD=y
 CONFIG_SPI_FLASH_ATMEL=y
 CONFIG_SPI_FLASH_EON=y
 CONFIG_SPI_FLASH_GIGADEVICE=y