arm/fvp-base-a-aem: add Architecture Envelope Model FVP for Arm v8-A

Add a recipe for the general purpose Armv-A Base RevC AEM FVP.

This FVP is behind a login-wall so the user has to download it themselves,
but the FVP is free to use.

Change-Id: I14d7b965a05ff9f405a2a12d4da2afd3dd2ecb87
Signed-off-by: Ross Burton <ross.burton@arm.com>