Enable secure memory support for FVPs

- Use the TrustZone controller on Base FVP to program DRAM access
  permissions. By default no access to DRAM is allowed if
  'secure memory' is enabled on the Base FVP.
- The Foundation FVP does not have a TrustZone controller but instead
  has fixed access permissions.
- Update FDTs for Linux to use timers at the correct security level.
- Starting the FVPs with 'secure memory' disabled is also supported.

Limitations:
Virtio currently uses a reserved NSAID. This will be corrected in
future FVP releases.

Change-Id: I0b6c003a7b5982267815f62bcf6eb82aa4c50a31