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/hsdk_defconfig b/configs/hsdk_defconfig
index ced0f6e..c0b7786 100644
--- a/configs/hsdk_defconfig
+++ b/configs/hsdk_defconfig
@@ -42,6 +42,7 @@
 CONFIG_MMC_DW=y
 CONFIG_MMC_DW_SNPS=y
 CONFIG_DM_SPI_FLASH=y
+CONFIG_MTD=y
 CONFIG_SPI_FLASH_SST=y
 CONFIG_DM_ETH=y
 CONFIG_ETH_DESIGNWARE=y