docs(maintainers): update AMD maintainers list

Maintainers for AMD platform ports has been updated.
"Amit Nagal" and "Akshay Belsare" are added to the list.

Change-Id: Ia64e1ec6c2f80515054730d307d41b0060d3dcc7
Signed-off-by: Akshay Belsare <akshay.belsare@amd.com>
1 file changed