firmware: scmi: mailbox/smt agent device

This change implements a mailbox transport using SMT format for SCMI
exchanges. This implementation follows the Linux kernel and
SCP-firmware [1] as references implementation for SCMI message
processing using SMT format for communication channel meta-data.

Use of mailboxes in SCMI FDT bindings are defined in the Linux kernel
DT bindings since v4.17.

Links: [1] https://github.com/ARM-software/SCP-firmware
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Cc: Simon Glass <sjg@chromium.org>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/firmware/scmi/Makefile b/drivers/firmware/scmi/Makefile
index 336ea1f..d22f53e 100644
--- a/drivers/firmware/scmi/Makefile
+++ b/drivers/firmware/scmi/Makefile
@@ -1,2 +1,4 @@
 obj-y	+= scmi_agent-uclass.o
+obj-y	+= smt.o
+obj-$(CONFIG_DM_MAILBOX)	+= mailbox_agent.o
 obj-$(CONFIG_SANDBOX)		+= sandbox-scmi_agent.o