The SSE-300 includes a counter module; implement a model of it.

This counter is 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: 20210219144617.4782-12-peter.maydell@linaro.org