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/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 99dc00f..a6e8d6e 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -48,3 +48,4 @@
 CONFIG_CLK=y
 CONFIG_RESET=y
 CONFIG_RAM=y
+CONFIG_DM_MMC=y