SUPPORT.md: Describe SCMI-SMC layer feature

Describe the layer which enables SCMI over SMC calls forwarding
to EL3 FW if issued by the Hardware domain. If the SCMI firmware
node is not found in the Host DT during initialization, it fails
silently as it's not mandatory.

The SCMI SMCs trapping at EL2 now lets hwdom perform SCMI ops for
interacting with system-level resources almost as if it would be
running natively.

Signed-off-by: Andrei Cherechesu <andrei.cherechesu@nxp.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Michal Orzel <michal.orzel@amd.com>

SUPPORT.md

@@ -935,6 +935,14 @@ Add/Remove device tree nodes using a device tree overlay binary (.dtbo).
 
     Status: Tech Preview
 
+### Arm: SCMI over SMC calls forwarding to EL3 Firmware
+
+Enable SCMI calls using SMC as doorbell mechanism and Shared Memory for
+transport ("arm,scmi-smc" compatible only) to reach EL3 Firmware if issued
+by hwdom. Some platforms use SCMI for access to system-level resources.
+
+    Status: Supported
+
 ## Virtual Hardware, QEMU
 
 This section describes supported devices available in HVM mode using a