infra: update the documentation for RD-INFRA-2022.08.18 release

The RD-INFRA-2022.08.18 release updates the software stack for RD-N2 and
variant reference design platforms. This release primarily updates the
RD-N2-Cfg2 platform software and adds support for building and booting
the software stack on AArch64 machines as well.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: If2684fea0eaf1a4a9174d5b8cc7751adcd27a95c