system-arm: Split the VExpress_GEM5_V1 base dts

system-arm: Split the VExpress_GEM5_V1 base dts

With the introduction of the new DPU model, we need different
variations of the VExpress_GEM5_V1 platform. This splits the platform
dtsi file into a separate file for the base platform and the
HDLCD-based platform. This matches the hierarchy in RealView.py.

Change-Id: Id02380122655b5d3aa3548a703fdef178bba17d9
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/11035
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>