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/helios4_defconfig b/configs/helios4_defconfig
index e38292e..d21e72a 100644
--- a/configs/helios4_defconfig
+++ b/configs/helios4_defconfig
@@ -51,6 +51,7 @@
 CONFIG_MMC_SDHCI=y
 CONFIG_MMC_SDHCI_SDMA=y
 CONFIG_MMC_SDHCI_MV=y
+CONFIG_MTD=y
 CONFIG_SF_DEFAULT_BUS=1
 CONFIG_SF_DEFAULT_SPEED=104000000
 CONFIG_SPI_FLASH_WINBOND=y