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/test/dm/blk.c b/test/dm/blk.c
index f4ea32e..012bf4c 100644
--- a/test/dm/blk.c
+++ b/test/dm/blk.c
@@ -83,12 +83,12 @@
 	ut_asserteq_ptr(usb_dev, dev_get_parent(dev));
 
 	/* Check we have one block device for each mass storage device */
-	ut_asserteq(3, count_blk_devices());
+	ut_asserteq(4, count_blk_devices());
 
 	/* Now go around again, making sure the old devices were unbound */
 	ut_assertok(usb_stop());
 	ut_assertok(usb_init());
-	ut_asserteq(3, count_blk_devices());
+	ut_asserteq(4, count_blk_devices());
 	ut_assertok(usb_stop());
 
 	return 0;