docs(commit-style): change blessed scope of FF-A

Also split SPM MM into it's own scope.

Change-Id: I9cfb1ddec7419ad0d7b539f65e7322bbd44a3913
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>
diff --git a/changelog.yaml b/changelog.yaml
index b591da3..3dee12b 100644
--- a/changelog.yaml
+++ b/changelog.yaml
@@ -446,10 +446,10 @@
 
     subsections:
       - title: FF-A
-        scope: ffa
+        scope: ff-a
 
         deprecated:
-          - ff-a
+          - ffa
 
       - title: RME
         scope: rme
@@ -461,7 +461,9 @@
           - spmc
           - spmd
           - SPMD
-          - spm_mm
+
+      - title: SPM MM
+        scope: spm-mm
 
   - title: Libraries