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/sandbox_mmc.c b/drivers/mmc/sandbox_mmc.c
new file mode 100644
index 0000000..f4646a8
--- /dev/null
+++ b/drivers/mmc/sandbox_mmc.c
@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2015 Google, Inc
+ * Written by Simon Glass <sjg@chromium.org>
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#include <common.h>
+#include <dm.h>
+#include <errno.h>
+#include <mmc.h>
+#include <asm/test.h>
+
+DECLARE_GLOBAL_DATA_PTR;
+
+static const struct udevice_id sandbox_mmc_ids[] = {
+	{ .compatible = "sandbox,mmc" },
+	{ }
+};
+
+U_BOOT_DRIVER(warm_mmc_sandbox) = {
+	.name		= "mmc_sandbox",
+	.id		= UCLASS_MMC,
+	.of_match	= sandbox_mmc_ids,
+};