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/am57xx_hs_evm_defconfig b/configs/am57xx_hs_evm_defconfig
index b3abbba..39c17b0 100644
--- a/configs/am57xx_hs_evm_defconfig
+++ b/configs/am57xx_hs_evm_defconfig
@@ -66,6 +66,7 @@
 CONFIG_SUPPORT_EMMC_BOOT=y
 CONFIG_MMC_OMAP_HS=y
 CONFIG_DM_SPI_FLASH=y
+CONFIG_MTD=y
 CONFIG_SF_DEFAULT_MODE=0
 CONFIG_SF_DEFAULT_SPEED=76800000
 CONFIG_SPI_FLASH_SPANSION=y