FVP: make usage of Trusted DRAM optional at build time

This patch groups the current contents of the Trusted DRAM region at
address 0x00_0600_0000 (entrypoint mailboxes and BL3-1 parameters) in
a single shared memory area that may be allocated to Trusted SRAM
(default) or Trusted DRAM at build time by setting the
FVP_SHARED_DATA_LOCATION make variable. The size of this shared
memory is 4096 bytes.

BLn_BASE and BLn_LIMIT values (mandatory as part of the porting) are
used to define the memory region for each linker script, instead of
the whole Trusted SRAM available in the platform. In FVP, these
definitions have been updated to accommodate the shared memory area
using the FVP_TRUSTED_SRAM_LIMIT definition.

TZROM_BASE/SIZE and TZRAM_BASE/SIZE are no longer required as part of
the platform porting. In FVP, they have been renamed to
FVP_TRUSTED_ROM_BASE/SIZE and FVP_TRUSTED_SRAM_BASE/SIZE.

FVP common definitions are defined in fvp_def.h while platform_def.h
contains exclusively (with a few exceptions) the definitions that are
mandatory in the porting guide. Therefore, platform_def.h now includes
fvp_def.h instead of the other way around.

The combination 'Shared data in Trusted SRAM + TSP in Trusted DRAM'
is not currently supported due to restrictions in the maximum number
of mmu tables that can be created.

Documentation has been updated to reflect these changes.

Fixes ARM-software/tf-issues#100

Change-Id: I26ff04d33ce4cacf8d770d1a1e24132b4fc53ff0