Minor modification to the README file

Qualify Base and Foundation FVPs as "Armv8 Architecture FVPs", to align
with the terminology used on this page:
https://developer.arm.com/products/system-design/fixed-virtual-platforms



Change-Id: Iabdcc37e0ef25373a3d4bb7d1ad964ea53e009a5
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>