infra: update release tag for all platforms

For the RD-INFRA-2021.09.17 release, the software stack for all the
Neoverse reference design platforms is rebased, revalidated and the
release tag is applied. So list this release tag in the documentation
for all the all the supported platforms.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: Ic69724ff4b573570c5c2527f74c3fc6013e2692f