hw/timer/imx_epit: fix compare timer handling

hw/timer/imx_epit: fix compare timer handling

- fix #1263 for CR writes
- rework compare time handling
  - The compare timer has to run even if CR.OCIEN is not set,
    as SR.OCIF must be updated.
  - The compare timer fires exactly once when the
    compare value is less than the current value, but the
    reload values is less than the compare value.
  - The compare timer will never fire if the reload value is
    less than the compare value. Disable it in this case.

Signed-off-by: Axel Heider <axel.heider@hensoldt.net>
[PMM: fixed minor style nits]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>