added SAMD51 support

[drewfish]

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This adds support for SAMD51 boards, both with and without crystal.

Tested with:

• Adafruit Feather M0 (SAMD21 w/crystal)
• Adafruit ItsyBitsy M4 (SAMD51 crystalless)
• SparkFun SAMD51 Thing Plus (SAMD51 w/crystal)

Tests included running the processors in standby mode.

Addresses https://github.com/arduino-libraries/RTCZero/issues/57

[drewfish]

I realize that all those "same for SAMD21 and SAMD51" comments might be considered noisy. I'm happy to remove them, or make any other style changes.

[dcelectr]

@drewfish THANK YOU! I tested the files from this pull request on Feather M0, Feather M4, ItsyBitsy M4, and Arduino Zero with the SimpleRTC.ino, and I had NO issues. (For Arduino ZERO I had to change Serial.* to SerialUSB.* in the SimpleRTC.ino sketch to view Serial monitor over the native USB.)

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