platforms/rdedmunds: add model startup support for rdedmunds platform

Add initial model startup support for RD-Edmunds platform. This supports
busybox, buildroot, distro boot, secure boot and acs tests.

Signed-off-by: Tony K Nadackal <tony.nadackal@arm.com>
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: I3e2698acb63dd0a68f9af25c4cd7b4c2cb222028