changelog: update release note per review feedback

- Update the changelog & release note per release note review guidance and
referencing on other projects' best practice.

Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Tianyu Li <tianyu.li@arm.com>
Reviewed-by: Willen Yang <Willen.Yang@arm.com>

Change-Id: Iee60e5f8fa8304340c083ed08a300e14fc413945