Update model support in User Guide

The CI has been updated to run tests against the AEMv8-A RevC
model, FVP_Base_RevC-2xAEMv8A, which is available from the Fast
Model releases on Connected Community [1].

Additionally, the CI now also includes the Cortex-A55x4, Cortex-A75x4
and Cortex-A55x4-A75x4 Base models.

[1] https://developer.arm.com/products/system-design/fixed-virtual-platforms



Change-Id: I57806f3b2a8121211490a7aa0089dcae566d8635
Signed-off-by: David Cunado <david.cunado@arm.com>