FVP: Allow BL3-2 to sit in the secure region of DRAM

FVP: Allow BL3-2 to sit in the secure region of DRAM

This patch allows the secure payload (BL3-2) to be loaded in the
DRAM region secured by the TrustZone controller (top 16 MB of DRAM1).

The location of BL3-2 can be selected at build time by setting the
build flag FVP_TSP_RAM_LOCATION to one of the following options:

  - 'tsram' : Trusted SRAM (this is the default option)
  - 'tdram' : Trusted DRAM
  - 'dram'  : Secure region in DRAM1 (top 16MB configured by the
              TrustZone controller)

The number of MMU tables in BL3-2 depends on its location in
memory: 3 in case it is loaded in DRAM, 2 otherwise.

Documentation updated accordingly.

Fixes ARM-software/tf-issues#212

Change-Id: I371eef3a4159f06a0c9e3c6c1f4c905b2f93803a