doc: release-notes-4.0: add release notes for firmware subsystem

Includes notes on SCMI's introduction.

(cherry picked from commit c0a0e6a)

Original-Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>
GitOrigin-RevId: c0a0e6a
Cr-Build-Id: 8731300272476715313
Cr-Build-Url: https://cr-buildbucket.appspot.com/build/8731300272476715313
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Change-Id: I72876f76fc589f0972e68daba3bba2a9cf2e9cc4
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/6021174
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Reviewed-by: Al Semjonovs <asemjonovs@google.com>
Tested-by: Al Semjonovs <asemjonovs@google.com>
Commit-Queue: Al Semjonovs <asemjonovs@google.com>

doc/releases/release-notes-4.0.rst

@@ -1209,6 +1209,14 @@ Libraries / Subsystems
 
     (:github:`79653`)
 
+* Firmware
+
+  * Introduced basic support for ARM's System Control and Management Interface, which includes:
+
+    * Subset of clock management protocol commands
+    * Subset of pin control protocol commands
+    * Shared memory and mailbox-based transport
+
 HALs
 ****