firmware: scmi: support Arm SMCCC transport

This change implements a SMCCC transport for SCMI exchanges. This
implementation follows the Linux kernel as references implementation
for SCMI message processing, using the SMT format for communication
channel meta-data.

Use of SMCCC transport in SCMI FDT bindings are defined in the Linux
kernel DT bindings since v5.8. SMCCC with SMT is implemented in OP-TEE
from tag 3.9.0 [2].

Links: [2] https://github.com/OP-TEE/optee_os/commit/a58c4d706d23
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 d22f53e..2f782bb 100644
--- a/drivers/firmware/scmi/Makefile
+++ b/drivers/firmware/scmi/Makefile
@@ -1,4 +1,5 @@
 obj-y	+= scmi_agent-uclass.o
 obj-y	+= smt.o
+obj-$(CONFIG_ARM_SMCCC) 	+= smccc_agent.o
 obj-$(CONFIG_DM_MAILBOX)	+= mailbox_agent.o
 obj-$(CONFIG_SANDBOX)		+= sandbox-scmi_agent.o