counter: stm32-timer-cnt: add counter prescaler extension

There's a prescaler in between the selected input signal used for
counting (CK_PSC), and the counter input (CK_CNT).
So add the "prescaler" extension to the counter.

Reviewed-by: William Breathitt Gray <william.gray@linaro.org>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20240307133306.383045-6-fabrice.gasnier@foss.st.com


Signed-off-by: William Breathitt Gray <william.gray@linaro.org>