The memory layout for the FPGA is fairly uniform for most of the FPGA
images, and we already assume that DRAM starts at 2GB by default.

Prepopulate PRELOADED_BL33_BASE and FPGA_PRELOADED_DTB_BASE to some
sane default values, to simplify building some stock image.
If people want to deviate from that, they can always override those
addresses on the make command line.

Change-Id: I2238fafb3f8253a01ad2d88d45827c141d9b29dd
Signed-off-by: Andre Przywara <andre.przywara@arm.com>