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/Kconfig b/drivers/mailbox/Kconfig
index 295e6db..9087512 100644
--- a/drivers/mailbox/Kconfig
+++ b/drivers/mailbox/Kconfig
@@ -10,4 +10,11 @@
 	  the basis of a variety of inter-process/inter-CPU communication
 	  protocols.
 
+config SANDBOX_MBOX
+	bool "Enable the sandbox mailbox test driver"
+	depends on DM_MAILBOX && SANDBOX
+	help
+	  Enable support for a test mailbox implementation, which simply echos
+	  back a modified version of any message that is sent.
+
 endmenu