samples: bbc_microbit: Align with recent changes in pwm_nrf5_sw driver

- add `channel-gpios` property with GPIO assignments for PWM channels
  to `sw_pwm` nodes
- update PWM related macros to refer to channels instead of pins
- remove no longer needed inclusion of boards/arm/bbc_microbit/board.h

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