xen: Add static event channel in SUPPORT.md on ARM

Static event channel support is tech preview, which shall be documented
in SUPPORT.md

Signed-off-by: Rahul Singh <rahul.singh@arm.com>
Reviewed-by: Ayan Kumar Halder <ayankuma@amd.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>

SUPPORT.md

@@ -922,6 +922,13 @@ bootscrub=off are passed as Xen command line parameters. (Memory should
 be scrubbed with bootscrub=idle.) No XSAs will be issues due to
 unscrubbed memory.
 
+## Static Event Channel
+
+Allow to setup the static event channel on dom0less system, enabling domains
+to send/receive notifications.
+
+    Status, ARM: Tech Preview
+
 # Format and definitions
 
 This file contains prose, and machine-readable fragments.