hw/misc/mps2_scc: Implement MPS2 Serial Communication Controller

hw/misc/mps2_scc: Implement MPS2 Serial Communication Controller

Implement a model of the Serial Communication Controller (SCC) found
in MPS2 FPGA images.

The primary purpose of this device is to communicate with the
Motherboard Configuration Controller (MCC) which is located on
the MPS board itself, outside the FPGA image. This is used
for programming the MPS clock generators. The SCC also has
some basic ID registers and an output for the board LEDs.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 1500029487-14822-7-git-send-email-peter.maydell@linaro.org