docs: update supported FVP models documentation

Update supported models list according to changes for v2.7 release in
ci/tf-a-ci-scripts repository:
* general FVP model update: 5c54251
* CSS model update: 3bd12fb

Signed-off-by: Maksims Svecovs <maksims.svecovs@arm.com>
Change-Id: I38c2ef2991b23873821c7e34ad2900b9ad023c4b