Feature Request: Add Support for SAMD51 MCUs

[TheShadowhawke]

Could support be added to this library for the SAMD51 series of MCUs? Thanks in advance.

[dcelectr]

@TheShadowhawke and @per1234
I tried this on the Feather M4, Metro M4, and Grand Central M4, and it worked on all three different boards.
https://github.com/ericbaril72/RTCZero/tree/SAMD51_E54
I installed the latest from Arduino library manager (current version 1.6.0) and overwrote only the RTCZero.cpp file.
This modified library did not work on ItsyBitsy M4 because it is crystal-less. It may be possible to add support for crystal-less SAMD51.

[drewfish]

I have a version which works on the Adafruit ItsyBitsy M4, I could make a pull request if repo owner(s) think it might be valuable. (I'd first want to test it out with SAMD21 crystal & crystalless boards also, of course.)