FVP: keep shared data in Trusted SRAM

This patch deprecates the build option to relocate the shared data
into Trusted DRAM in FVP. After this change, shared data is always
located at the base of Trusted SRAM. This reduces the complexity
of the memory map and the number of combinations in the build
options.

Fixes ARM-software/tf-issues#257

Change-Id: I68426472567b9d8c6d22d8884cb816f6b61bcbd3
Merge pull request #217 from jcastillo-arm/jc/tf-issues/257

FVP: keep shared data in Trusted SRAM