arm_fpga: Add devicetree file

The FPGA images used in Arm Ltd. focus on CPU cores, so they share a
common platform, with a minimal set of peripherals (interconnect, GIC,
UART).
This allows to support most platforms with a single devicetree file.
The topology and number of CPU cores differ, but those will added at
runtime, in BL31. Other adjustments (GICR size, SPE node, command line)
are also done at this point.

Add the common devicetree file to TF-A's build system, so it can be
build together with BL31. At runtime, the resulting .dtb file should be
uploaded to the address given with FPGA_PRELOADED_DTB_BASE at build time.

Change-Id: I3206d6131059502ec96896e95329865452c9d83e
Signed-off-by: Andre Przywara <andre.przywara@arm.com>