hw/misc/iotkit-secctl: Arm IoT Kit security controller initial skeleton

The Arm IoT Kit includes a "security controller" which is largely a
collection of registers for controlling the PPCs and other bits of
glue in the system.  This commit provides the initial skeleton of the
device, implementing just the ID registers, and a couple of read-only
read-as-zero registers.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180220180325.29818-16-peter.maydell@linaro.org