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

For the RD-INFRA-2021.04.30 release, update the RD-N2 and RD-N2-Cfg1
platform manifest. The only change in the manifest of both of these
platforms is the tag used, that is, tag is changed to
"RD-INFRA-2021.04.30".

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I99dbd200d2796e9ab1a400bdaa2c927c913c287d