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/boston64r2el_defconfig b/configs/boston64r2el_defconfig
index d8657a3..aa07539 100644
--- a/configs/boston64r2el_defconfig
+++ b/configs/boston64r2el_defconfig
@@ -32,6 +32,7 @@
 CONFIG_ENV_ADDR=0xFFFFFFFFBFFE0000
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_CLK=y
+CONFIG_MTD=y
 CONFIG_DM_MTD=y
 CONFIG_MTD_NOR_FLASH=y
 CONFIG_FLASH_CFI_DRIVER=y