drivers: pwm: fix esp32-s3 ledc pwm low frequency

ESP32 - S2,S3 and C3 variants have only 14 bits counter.
where as the plain ESP32 variant has 20 bits counter.
application failed to set low frequency(1Hz) in S2, S3 and C3 variants.
to get very low frequencies on these variants,
frequency needs to be tuned with 18 bits clock divider.
so select the slow clock source (1MHz) with highest counter resolution.
this can be handled on the func'pwm_led_esp32_timer_set' with 'prescaler'.

Signed-off-by: Jeeva Kandasamy <jkandasa@gmail.com>