boards: nrf: Align with recent changes made to pwm_nrfx driver

... and "nordic,nrf-pwm" binding:

- use channel indexes instead of pin numbers in `pwms` properties that
  define PWM LEDs
- add the period and flags cells to `pwms` properties in all PWM LED
  definitions; use the commonly used period of 20 ms (giving 50 Hz)
  as a default setting

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>