hw/timer/imx_epit: remove explicit fields cnt and freq

hw/timer/imx_epit: remove explicit fields cnt and freq

The CNT register is a read-only register. There is no need to
store it's value, it can be calculated on demand.
The calculated frequency is needed temporarily only.

Note that this is a migration compatibility break for all boards
types that use the EPIT peripheral.

Signed-off-by: Axel Heider <axel.heider@hensoldt.net>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>