The MPS2 AN505 FPGA image includes a "FPGA control block"
which is a small set of registers handling LEDs, buttons
and some counters.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180220180325.29818-14-peter.maydell@linaro.org