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

arch/sandbox/include/asm/mbox.h

@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2016, NVIDIA CORPORATION.
+ *
+ * SPDX-License-Identifier: GPL-2.0
+ */
+
+#ifndef __SANDBOX_MBOX_H
+#define __SANDBOX_MBOX_H
+
+#include <common.h>
+
+#define SANDBOX_MBOX_PING_XOR 0x12345678
+
+struct udevice;
+
+int sandbox_mbox_test_get(struct udevice *dev);
+int sandbox_mbox_test_send(struct udevice *dev, uint32_t msg);
+int sandbox_mbox_test_recv(struct udevice *dev, uint32_t *msg);
+int sandbox_mbox_test_free(struct udevice *dev);
+
+#endif