counter: stm32-timer-cnt: introduce clock signal

Introduce the internal clock signal, used to count when in simple rising
function. Also add the "frequency" extension to the clock signal.

With this patch, signal action reports a consistent state when "increase"
function is used, and the counting frequency:
    $ echo increase > function
    $ grep -H "" signal*_action
    signal0_action:none
    signal1_action:none
    signal2_action:rising edge
    $ echo 1 > enable
    $ cat count
    25425
    $ cat count
    44439
    $ cat ../signal2/frequency
    208877930

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20240307133306.383045-5-fabrice.gasnier@foss.st.com


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