hw/misc/iotkit-sysctl: Implement IoTKit system control element

hw/misc/iotkit-sysctl: Implement IoTKit system control element

The Arm IoTKit includes a system control element which
provides a block of read-only ID registers and a block
of read-write control registers. Implement a minimal
version of this.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180820141116.9118-9-peter.maydell@linaro.org