infra/change-log: update change log for RD-INFRA-2021.08.11 refresh

Update change log for the RD-INFRA-2021.08.11 refresh. The platforms
that are receiving software updates are RD-N2 and RD-N2-Cfg1. Document
the changes in the software stack for these two platforms.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I222b82370437ba588a357b1e509ef347d426a666