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>