Docs: Release notes for v2.2.0

- Add notes for TF-Mv2.2.0.
- Remove refs to v1.8.x from TOC to simplify the view, though the
  documents are not removed.

Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Change-Id: Ic58768114305f89425c7e58a440b5b29787842e0
7 files changed