fix(docs): update the v2.6 change-log

Updated the v2.6 change-log for below:
1. Moved ETE/ETM related changes under separate scope
2. Added manually commit log for Demeter CPU

Change-Id: Ib5b5f994f603af6c82b1400256752581a7931268
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
2 files changed