The FVP default configuration has bp.dram_size=4, which is sufficient
for development and testing purposes, so remove the FVP_CONFIG
override and set to 4 Gb in the device tree.

Issue-Id: SCM-3871
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: I4a96062c9e94d36f5459f33c86aab4d4885bab43
Signed-off-by: Jon Mason <jon.mason@arm.com>