hw/timer/cmsdk-apb-timer: run or stop timer on writes to RELOAD and VALUE

If the CMSDK APB timer is set up with a zero RELOAD value
then it will count down to zero, fire once and then stay
at zero. From the point of view of the ptimer system, the
timer is disabled; but the enable bit in the CTRL register
is still set and if the guest subsequently writes to the
RELOAD or VALUE registers this should cause the timer to
start counting down again.

Add code to the write paths for RELOAD and VALUE so that
we correctly restart the timer in this situation.

Conversely, if the new RELOAD and VALUE are both zero,
we should stop the ptimer.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Message-id: 20180703171044.9503-5-peter.maydell@linaro.org