mailbox: implement a sandbox test

This adds a sandbox mailbox implementation (provider), a test client
device, instantiates them both from Sandbox's DT, and adds a DM test
that excercises everything.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org> # v1
diff --git a/test/dm/mailbox.c b/test/dm/mailbox.c
new file mode 100644
index 0000000..be7bd6d
--- /dev/null
+++ b/test/dm/mailbox.c
@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2016, NVIDIA CORPORATION.
+ *
+ * SPDX-License-Identifier: GPL-2.0
+ */
+
+#include <common.h>
+#include <dm.h>
+#include <dm/test.h>
+#include <asm/mbox.h>
+#include <test/ut.h>
+
+static int dm_test_mailbox(struct unit_test_state *uts)
+{
+	struct udevice *dev;
+	uint32_t msg;
+
+	ut_assertok(uclass_get_device_by_name(UCLASS_MISC, "mbox-test", &dev));
+	ut_assertok(sandbox_mbox_test_get(dev));
+
+	ut_asserteq(-ETIMEDOUT, sandbox_mbox_test_recv(dev, &msg));
+	ut_assertok(sandbox_mbox_test_send(dev, 0xaaff9955UL));
+	ut_assertok(sandbox_mbox_test_recv(dev, &msg));
+	ut_asserteq(msg, 0xaaff9955UL ^ SANDBOX_MBOX_PING_XOR);
+	ut_asserteq(-ETIMEDOUT, sandbox_mbox_test_recv(dev, &msg));
+
+	ut_assertok(sandbox_mbox_test_free(dev));
+
+	return 0;
+}
+DM_TEST(dm_test_mailbox, DM_TESTF_SCAN_FDT);