arm64: dts: split Foundation model dts to put the GIC separately

The ARMv8 Foundation model can be run with a GICv2 or a GICv3.
To prepare for the GICv3 version of the .dts without code duplication,
move most of the nodes of the existing DT (except the GIC) into an
include file and just keep that include statement and the GIC node in
the current foundation-v8.dts.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>