hw/timer/cmsdk-apb-timer: Correctly identify and set one-shot mode

The CMSDK APB timer is currently always configured as periodic timer.
This results in the following messages when trying to boot Linux.

Timer with delta zero, disabling

If the timer limit set with the RELOAD command is 0, the timer
needs to be enabled as one-shot timer.

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