hw/timer/cmsdk-apb-timer: Correct ptimer policy settings

The CMSDK timer interrupt triggers when the counter goes from 1 to 0,
so we want to trigger immediately, rather than waiting for a
clock cycle. Drop the incorrect NO_IMMEDIATE_TRIGGER setting.
We also do not want to get an interrupt if the guest sets the
counter directly to zero, so use the new TRIGGER_ONLY_ON_DECREMENT
policy.

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: 20180703171044.9503-3-peter.maydell@linaro.org