Add Linux DTS for FVP with threaded CPUs

Add Linux DTS for FVP with threaded CPUs

In contrast with the non-multi-threading DTS, this enumerates MPIDR
values shifted by one affinity level to the left. The newly added DTS
reflects CPUs with a single thread in them.

Since both DTS files are the same apart from MPIDR contents, the common
bits have been moved to a separate file that's then included from the
top-level DTS files. The multi-threading version only updates the MPIDR
contents.

Change-Id: Id225cd93574f764171df8962ac76f42fcb6bba4b
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>