samd/machine_pwm: Add the machine.PWM class.

Features are:
- 3 to 5 different frequency groups.
- Freq range of 1Hz - 24 MHz.
- Duty rate stays stable on freq change.

Keyword options to the PWM constructor:
- device=n Select a specific PWM device.  If no device is specified, a free
           device is chosen, if available at that pin.
- freq=nnnn
- duty_u16=nnnn
- duty_ns=nnnn
- invert=True/False Allowing two outputs on the same device/channel to have
                    complementary signals.

If both freq and duty are provided, PWM output will start immediately.

Pins at the same device have the same frequency.  If the PWM output number
exceeds the number of channels at the PWM device, the effctive channel_no
is output_no % channel_count.  So with a channel count of 4, output 7 is
assigned to channel 3.  Pins at a certain channel have the same frequency
and duty rate, but may be seperately inverted.

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