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.

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
diff --git a/plat/fvp/aarch64/fvp_common.c b/plat/fvp/aarch64/fvp_common.c
index b8f32a1..5041511 100644
--- a/plat/fvp/aarch64/fvp_common.c
+++ b/plat/fvp/aarch64/fvp_common.c
@@ -56,8 +56,8 @@
  * configure_mmu_elx() will give the available subset of that,
  */
 const mmap_region_t fvp_mmap[] = {
-	{ FVP_TRUSTED_ROM_BASE,	FVP_TRUSTED_ROM_BASE,	FVP_TRUSTED_ROM_SIZE,
-						MT_MEMORY | MT_RO | MT_SECURE },
+	{ FVP_SHARED_RAM_BASE,	FVP_SHARED_RAM_BASE,	FVP_SHARED_RAM_SIZE,
+						MT_MEMORY | MT_RW | MT_SECURE },
 	{ FVP_TRUSTED_DRAM_BASE, FVP_TRUSTED_DRAM_BASE,	FVP_TRUSTED_DRAM_SIZE,
 						MT_MEMORY | MT_RW | MT_SECURE },
 	{ FLASH0_BASE,	FLASH0_BASE,	FLASH0_SIZE,