docs(maintainers): update AMD-Xilinx Maintainers

Update "Xilinx" platform subheader with "AMD-Xilinx", to reuse
the maintainer list for legacy Xilinx platforms and
upcoming AMD platforms.
Update maintainer list for AMD-Xilinx platforms in TF-A repository.

Change-Id: I3ea81d36eb8bb5269fe69ca5362800dc309b21c1
Signed-off-by: Akshay Belsare <akshay.belsare@amd.com>
1 file changed