FVP: Add support for passing platform's topology to DTS ...

This patch adds support for passing FVP platform's topology
configuration to DTS files for compilation, which allows to
build DTBs with correct number of clusters and CPUs.
This removes non-existing clusters/CPUs from the compiled
device tree blob and fixes reported Linux errors when trying
to power on absent CPUs/PEs.
If DTS file is passed using FVP_HW_CONFIG_DTS build option from
the platform's makefile, FVP_CLUSTER_COUNT, FVP_MAX_CPUS_PER_CLUSTER
and FVP_MAX_PE_PER_CPU parameters are used, otherwise CI script will
use the default values from the corresponding DTS file.

Change-Id: Idcb45dc6ad5e3eaea18573aff1a01c9344404ab3
Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>