hw/arm/iotkit: Refactor into abstract base class and subclass

hw/arm/iotkit: Refactor into abstract base class and subclass

The Arm SSE-200 Subsystem for Embedded is a revised and
extended version of the older IoTKit SoC. Prepare for
adding a model of it by refactoring the IoTKit code into
an abstract base class which contains the functionality,
driven by a class data block specific to each subclass.
(This is the same approach used by the existing bcm283x
SoC family implementation.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190121185118.18550-6-peter.maydell@linaro.org