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/boston32r6_defconfig b/configs/boston32r6_defconfig
index 7e33423..0fffc46 100644
--- a/configs/boston32r6_defconfig
+++ b/configs/boston32r6_defconfig
@@ -31,6 +31,7 @@
 CONFIG_ENV_ADDR=0xBFFE0000
 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