samples: various servo_motor fixes

Though there were issues with this sample before e959386b ("samples:
servo_motor: cleanups and changes"), that commit introduced further
bugs. This happened because the new pwm-servo alias that commit
switched to wasn't provided by any boards, so it wasn't built in CI.

Before that, however, the recommendation to use bbc_microbit in the
sample documentation was also buggy in a couple of ways:

    1. bbc_microbit doesn't have the pwm-0 alias the sample
       previously required, so it didn't build on that board

    2. the documentation's comment to use pin 0 on the edge connector
       is wrong; PWM channel 0 is wired to GPIO P0.0 on the SoC,
       which is actually pin 21 on the connector

Fix it all up.

Tested on bbc_microbit. I verified the pinout and also made sure that
the sample correctly generates pulses from 700 to 2300 usec.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>