aemfvp-a-rme: add pinned and un-pinned manifests

For the AEMFVP-A platform that supports RME, add two manifests. The
pinned manifest points to a known good integration of the software stack
of as the AEMFVP-A-RME-2023.03.17 release. The un-pinned manifest points
to active development branches and allows a more recent version of the
software stack to be used. But the un-pinned manifest might not provide
a stable working version of the software stack all the time.

Signed-off-by: Nuno Lopes <nuno.lopes@arm.com>
Change-Id: I31a3c1293fa32b450fb68412ae2674583db17a69