Add a simple test of the CMSDK APB timer, since we're about to do
some refactoring of how it is clocked.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210128114145.20536-4-peter.maydell@linaro.org
Message-id: 20210121190622.22000-4-peter.maydell@linaro.org