samples: servo_motor: Align with recent changes in pwm_nrf5_sw driver

- add `channel-gpios` property with GPIO assignment for the used PWM
  channel to the `sw_pwm` node
- replace ambiguous "pin 21" in the sample's README with "pin P19"
  that uses notation from the official micro:bit documentation and
  is consistent with this pin number within the edge_connector node

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