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/ap152_defconfig b/configs/ap152_defconfig
index 76181eb..29a782c 100644
--- a/configs/ap152_defconfig
+++ b/configs/ap152_defconfig
@@ -36,6 +36,7 @@
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 # CONFIG_NET is not set
 CONFIG_DM_SPI_FLASH=y
+CONFIG_MTD=y
 CONFIG_SPI_FLASH_BAR=y
 CONFIG_SPI_FLASH_WINBOND=y
 CONFIG_SPI_FLASH_DATAFLASH=y