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/drivers/mailbox/Makefile b/drivers/mailbox/Makefile
index a2d96a4..bbae4de 100644
--- a/drivers/mailbox/Makefile
+++ b/drivers/mailbox/Makefile
@@ -3,3 +3,5 @@
 # SPDX-License-Identifier: GPL-2.0
 
 obj-$(CONFIG_DM_MAILBOX) += mailbox-uclass.o
+obj-$(CONFIG_SANDBOX_MBOX) += sandbox-mbox.o
+obj-$(CONFIG_SANDBOX_MBOX) += sandbox-mbox-test.o