Model the ARM MPS2/MPS2+ FPGA based development board.

The MPS2 and MPS2+ dev boards are FPGA based (the 2+ has a bigger
FPGA but is otherwise the same as the 2). Since the CPU itself
and most of the devices are in the FPGA, the details of the board
as seen by the guest depend significantly on the FPGA image.

We model the following FPGA images:
 "mps2_an385" -- Cortex-M3 as documented in ARM Application Note AN385
 "mps2_an511" -- Cortex-M3 'DesignStart' as documented in AN511

They are fairly similar but differ in the details for some
peripherals.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1500029487-14822-2-git-send-email-peter.maydell@linaro.org
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>