Update AEM and Cortex Models versions

AEMv8-A Model release v8.4 has been made available and Trusted Firmware
has been tested against these versions as part of its CI system. This
patch updates the user guide documentation to reflect the version of AEM
and Cortex Models that Trusted Firmware has been tested against.

Additionally, ARM FVPs FVP_Base_Cortex-A57x1-A53x1 and
FVP_Base_Cortex-A57x2-A53x4 are removed from the list of tested FVPs
as they are currently not being tested with the latest version of ARM
Trusted Firmware.

Also, documentation and links to Linaro pages have been updated to
reflect the changes in the ARM community document hosting.

Change-Id: Idae97303ce0929c82b137017de84ce94678f6f2b
Signed-off-by: David Cunado <david.cunado@arm.com>