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: I9df71daa9c6fe7f658ba1d2c9d30a82154be8d7d
Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>