Embed custom PWM frequency logic

[VGavara]

Right now custom PWM frequency on SAMD21 based hardware is being managed using the SAMD21 Turbo PWM library. It allowed a fast integration of custom PWM features, but also arises some problems:

• Part of the library code is not being used
• Resolution is limite to 1000 steps, though ArduinoTB6612FNG supports resolutions up to 16 bit and, moreover, the resolutions under 23kHz frequencies are >= 2048 steps (see Wiki).
• Imposes a library dependency

The SAMD21 Turbo PWM library strictly relevant code must be extracted, simplified and stored in a new include file. Documentation making reference to the SAMD21 Turbo PWM dependency must be updated.