boards: arm: nucleo_f042k6: fix pwm output

Changes:
* Fixed typo in the PWM channel number (32 -> 3)
* Added a prescaler to make the board compatible
  with the blinky_pwm sample

Output of the sample before the fix:

PWM-based blinky
Calibrating for channel 32...
[00:00:00.010,000] <err> pwm_stm32: Invalid channel (32)
[00:00:00.016,000] <err> pwm_stm32: Invalid channel (32)
[00:00:00.022,000] <err> pwm_stm32: Invalid channel (32)
[00:00:00.028,000] <err> pwm_stm32: Invalid channel (32)
[00:00:00.034,000] <err> pwm_stm32: Invalid channel (32)
[00:00:00.040,000] <err> pwm_stm32: Invalid channel (32)
Error: PWM device does not support a period at least 31250000

After the fix:

PWM-based blinky
Calibrating for channel 3...
Done calibrating; maximum/minimum periods 1000000000/7812500 nsec

Presence of PWM signal after the fix
has been confirmed using a logic analyzer.

Signed-off-by: Maksim Salau <maksim.salau@gmail.com>