Modelled Microsemi's Smartfusion2 SPI controller.

Signed-off-by: Subbaraya Sundeep <sundeep.lkml@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20170920201737.25723-4-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>