refactor(fvp): fdts: consolidate GICv2 base FVP DT files

refactor(fvp): fdts: consolidate GICv2 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.
Since the GICv3 versions now use a generic DT include file (without any
GIC node), let's reuse that for the GICv2 versions of the FVP as well.
We just add a separate fvp-base-gicv2.dtsi file which describes the
GICv2 interrupt controller. Also shorten the compatible string, since
the GICv2 binding documentation does not allow the current combination.

This allows to remove the mostly redundant nodes from the GICv2 .dts
file.

Change-Id: I9018031bb611fb00ca7dbefc1bff7d40c3f05819
Signed-off-by: Andre Przywara <andre.przywara@arm.com>