MLECO-4826: Targeting Arm Corstone-315 boot ROM

* Updating linker scripts + scatter files to utilise
  64KiB boot region of Corstone-315.
* Now target 3 loading regions - boot, bram and dram.

Change-Id: Ieb9f03e53d913ded4facc15ab2171ce647b405e5
Signed-off-by: Alex Tawse <alex.tawse@arm.com>
Tested-by: mlecosys <mlecosys@arm.com>
Reviewed-by: Kshitij Sisodia <kshitij.sisodia@arm.com>
Signed-off-by: Kshitij Sisodia <kshitij.sisodia@arm.com>