Some recent enhancements to EL3 runtime firmware like support for
save and restoring GICv3 register context during system_suspend
necessitates additional data memory for the firmware. This patch
introduces support for creating a TZC secured DDR carveout for use
by ARM reference platforms. A new linker section `el3_tzc_dram` is
created using platform supplied linker script and data marked with
the attribute `arm_el3_tzc_dram` will be placed in this section.
The FVP makefile now defines the `PLAT_EXTRA_LD_SCRIPT` variable to
allow inclusion of the platform linker script by the top level BL31
linker script.

Change-Id: I0e7f4a75a6ac51419c667875ff2677043df1585d
Signed-off-by: Soby Mathew <soby.mathew@arm.com>