samples: bluetooth: mesh_demo: Align with changes in pwm_nrf5_sw driver

- add an overlay file for the bbc_microbit board with specification
  of GPIO that should be used as the PWM output to buzzer
- update a PWM related macro in the microbit specific code to refer
  to PWM channel instead of pin

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