vexpress64: Refactor header file to make it easier to add new FVPs

Rename from vexpress_aemv8a.h -> vepxress_aemv8.h as new FVPs may not be
v8-A. No change in behavior.

This is towards future work to enable support for the FVP_BaseR.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>