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/linkit-smart-7688_defconfig b/configs/linkit-smart-7688_defconfig
index 1ecd4df..28c8be2 100644
--- a/configs/linkit-smart-7688_defconfig
+++ b/configs/linkit-smart-7688_defconfig
@@ -45,6 +45,7 @@
 CONFIG_LED=y
 CONFIG_LED_BLINK=y
 CONFIG_LED_GPIO=y
+CONFIG_MTD=y
 CONFIG_SPI_FLASH_GIGADEVICE=y
 CONFIG_SPI_FLASH_MACRONIX=y
 CONFIG_SPI_FLASH_SPANSION=y