refactor(fvp): fdts: consolidate GICv3 base FVP DT files

The GICv2 and GICv3 version of the FVP DT files are unnecessarily split,
as the common part of the peripherals is the same: it's literally just
the interrupt controller node that is different.
To facilitate a unification, refactor the DT include files to explicitly
include a snippet with just the GICv3 description, and a generic base DT
file for the rest. This generic file can then be reused by the GICv2
versions later.

Since we can only have a /memreserve/ entry *before* any DT nodes, move
that line to each file, to allow including the GIC DT file separately.

Change-Id: I9ff357d3fe0ce46e280c30131aeae97a99631512
Signed-off-by: Andre Przywara <andre.przywara@arm.com>