counter: stm32-timer-cnt: introduce channels

counter: stm32-timer-cnt: introduce channels

Simply add channels 3 and 4 that can be used for capture. Statically
add them, despite some timers doesn't have them. Rather rely on
stm32_action_read that will report "none" action for these currently.

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/20240307133306.383045-8-fabrice.gasnier@foss.st.com
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>