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/ap121_defconfig b/configs/ap121_defconfig
index d658541..f509fa0 100644
--- a/configs/ap121_defconfig
+++ b/configs/ap121_defconfig
@@ -39,6 +39,7 @@
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_DM_SPI_FLASH=y
+CONFIG_MTD=y
 CONFIG_SPI_FLASH_ATMEL=y
 CONFIG_SPI_FLASH_EON=y
 CONFIG_SPI_FLASH_GIGADEVICE=y