Add plat-vexpress

* Initial support for Versatile Express of FVP with ARM Trusted
  Firmware (ATF below)

* Use entry vector for entering TEE

  Align interface between TEE and secure monitor with the interface
  between OPTEED in ATF and TEE.

  Uses an ATF compatible entry vector for entering TEE from internal
  secure monitor.

  Internal secure monitor saves entry reason to be able to tell when
  switching back to nonsecure world if r0-r3 should be preserved (FIQ case)
  or returned as is (normal call case).

* Many small fixes of generic problems that could affect other platforms
  too.

* Disable unaligned data accesses by adding compiler flag
  -mno-unaligned-access

* Adds support for Versatile Express of QEMU with 8 MiB of secure DRAM