hw/timer/sse-timer: Model the SSE Subsystem System Timer

The SSE-300 includes some timers which are a different kind to
those in the SSE-200. Model them.

These timers are documented in the SSE-123 Example Subsystem
Technical Reference Manual:
 https://developer.arm.com/documentation/101370/latest/



Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210219144617.4782-13-peter.maydell@linaro.org