Warn when PWM channels are reused

[ArendJan]

Warn the user when the same pwm channels are used for multiple pins/motors.

This config would give warnings:

motor:
  left_front:
    name: left_front
    device: mirte
    type: pp
    pins:
      p1: 18
      p2: 19
  right_rear:
    name: right_rear
    device: mirte
    type: pp
    pins:
      p1: 3
      p2: 2

as 2&3 have channel 1A&1B. 18&19 have the same channels. Driving around with these pins does not work