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/Kconfig b/drivers/firmware/scmi/Kconfig
index c501bf4..c3a109b 100644
--- a/drivers/firmware/scmi/Kconfig
+++ b/drivers/firmware/scmi/Kconfig
@@ -2,7 +2,7 @@
 	bool "Enable SCMI support"
 	select FIRMWARE
 	select OF_TRANSLATE
-	depends on SANDBOX || DM_MAILBOX
+	depends on SANDBOX || DM_MAILBOX || ARM_SMCCC
 	help
 	  System Control and Management Interface (SCMI) is a communication
 	  protocol that defines standard interfaces for power, performance
@@ -15,5 +15,5 @@
 
 	  Communications between agent (client) and the SCMI server are
 	  based on message exchange. Messages can be exchange over tranport
-	  channels as a mailbox device with some piece of identified shared
-	  memory.
+	  channels as a mailbox device or an Arm SMCCC service with some
+	  piece of identified shared memory.