SAMD51 support

[ladyada]

now working on G19 also tested on J19. We're using TC0 which is always available

[ladyada]

hiya FYI for people looking at this, we weren't sure if/when this'd be merged so we now include Servo as a library in the adafruit samd distribution for samd51 usage. If you update to the latest BSP you'll get it :)

[stonehippo]

It’s been a while since this PR was submitted. Any chance it could get merged in soon? It sort of suck having to use a different library to make Servo work on SAMD21/SAMD51 boards.

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❌ ladyada
❌ PaintYourDragon

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