dm: test: Add a test for the mmc uclass

Add a test to confirm that we can probe this device. Since there is no
MMC stack support in sandbox at present, this is as far as the test goes.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mmc/Makefile b/drivers/mmc/Makefile
index 2680c63..286df2f 100644
--- a/drivers/mmc/Makefile
+++ b/drivers/mmc/Makefile
@@ -31,6 +31,7 @@
 obj-$(CONFIG_SUPPORT_EMMC_RPMB) += rpmb.o
 obj-$(CONFIG_S3C_SDI) += s3c_sdi.o
 obj-$(CONFIG_S5P_SDHCI) += s5p_sdhci.o
+obj-$(CONFIG_SANDBOX) += sandbox_mmc.o
 obj-$(CONFIG_SDHCI) += sdhci.o
 obj-$(CONFIG_SH_MMCIF) += sh_mmcif.o
 obj-$(CONFIG_SH_SDHI) += sh_sdhi.o