build(changelog): add new scope for Performance Monitor Extensions

This patch adds a news scope for FEAT_PMUV3, alongside
updating the existing comments related to the saving of
PMCR_EL0 register routine for better understanding.

Change-Id: Ib150244ce94cfcbbe5d12fdae56327c3d72bda0b
Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>
diff --git a/changelog.yaml b/changelog.yaml
index d3e235d..8dbdd50 100644
--- a/changelog.yaml
+++ b/changelog.yaml
@@ -128,6 +128,9 @@
       - title: Trapping support for RNDR/RNDRRS (FEAT_RNG_TRAP)
         scope: rng-trap
 
+      - title: Performance Monitors Extension (FEAT_PMUv3)
+        scope: pmu
+
   - title: Platforms
 
     subsections: