Add a simple qtest to exercise the new system counter device in the
SSE-300.

We'll add tests of the system timer device here too, so this includes
scaffolding (register definitions, etc) for those.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210219144617.4782-45-peter.maydell@linaro.org