infra/rdn2: update rdn2 documentation for RD-INFRA-2021.04.30 release

The RD-INFRA-2021.04.30 release provide couple of updates in the
platform software for RD-N2 and RD-N2-Cfg1 platforms. Correspondingly,
update the platform documentation with the supported features and the
new tag to use.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I7a9db17aa14654df7e2626edaa2814a8877ccf70