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