dm: sandbox: mmc: Enable building MMC code for sandbox

Enable building the MMC code for sandbox. This increases build
coverage for sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index afdf4a3..aec2e53 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -1,4 +1,5 @@
 CONFIG_SYS_MALLOC_F_LEN=0x2000
+CONFIG_MMC=y
 CONFIG_PCI=y
 CONFIG_DEFAULT_DEVICE_TREE="sandbox"
 CONFIG_I8042_KEYB=y
@@ -97,6 +98,7 @@
 CONFIG_SPL_PWRSEQ=y
 CONFIG_RESET=y
 CONFIG_DM_MMC=y
+CONFIG_SANDBOX_MMC=y
 CONFIG_SPI_FLASH_SANDBOX=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_ATMEL=y