mtd: ensure MTD is compiled when CMD_MTDPARTS is selected

MTD support must be enabled when using mtdparts. Indeed, functions
like get_mtd_info(), get_mtd_device() and put_mtd_device() are in
drivers/mtd/mtd_uboot.c and are built only with CONFIG_MTD.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index eda595f..ed7ff78 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -136,6 +136,7 @@
 CONFIG_SPL_PWRSEQ=y
 CONFIG_I2C_EEPROM=y
 CONFIG_MMC_SANDBOX=y
+CONFIG_MTD=y
 CONFIG_SPI_FLASH_SANDBOX=y
 CONFIG_SPI_FLASH_ATMEL=y
 CONFIG_SPI_FLASH_EON=y