boards: arm: Add board for MPS2 with AN383

ARM's Cortex-M Prototyping System (MPS2) is a board with an FPGA that
can be programmed with different 'SoCs'. To use these in Zephyr we need
a set of board files for each variant.

This adds a board for a variant which implements a Cortex-M3 CPU; the
naming of this matches that used for the Zephyr SoC (which is itself
based on ARM's documentation nomenclature).

Change-Id: Ie02a67a03016b8aeee31e3694f0edbcc37f9ee64
Signed-off-by: Jon Medhurst <tixy@linaro.org>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>