Doc updates following platform port reorganization
Dan Handley authored
Update the User Guide, Porting Guide and Firmware Design documents
to align them with the recent changes made to the FVP and Juno
platform ports.

Also fix some other historical inaccuracies.

Change-Id: I37aba4805f9044b1a047996d3e396c75f4a09176
4a75b84a