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/ds109_defconfig b/configs/ds109_defconfig
index 2417990..01cafb6 100644
--- a/configs/ds109_defconfig
+++ b/configs/ds109_defconfig
@@ -33,6 +33,7 @@
 CONFIG_SYS_I2C_MVTWSI=y
 # CONFIG_MMC is not set
 CONFIG_DM_SPI_FLASH=y
+CONFIG_MTD=y
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_MVGBE=y
 CONFIG_MII=y