added SAMD51 support

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.

Author: drewfish

README.adoc

@@ -1,6 +1,9 @@
 = RTC Library for Arduino =
 
-The RTC library enables an Arduino Zero or MKR1000 board to take control of the internal RTC.
+The RTC library enables a supported board to take control of the internal RTC.
+Supported boards are:
+SAMD21-based boards (Arduino Zero, MKR1000, Adafruit Feather M0, Sparkfun SAMD21 Mini Breakout, etc),
+SAMD51-based boards (Adafruit Grand Central M4, SparkFun SAMD51 Thing Plus, etc).
 
 For more information about this library please visit us at
 http://arduino.cc/en/Reference/RTC

library.properties

@@ -2,8 +2,8 @@ name=RTCZero
 version=1.6.0
 author=Arduino
 maintainer=Arduino <[email protected]>
-sentence=Allows to use the RTC functionalities. For Arduino Zero, MKRZero and MKR1000 only.
-paragraph=With this library you can use the RTC peripheral of an Arduino Zero or MKR1000 in order to program actions related to date and time.
+sentence=Allows to use the RTC functionalities. For Arduino Zero, MKRZero, MKR1000 (SAMD21), and SAMD51 boards.
+paragraph=With this library you can use the RTC peripheral of a supported board in order to program actions related to date and time. Supported boards are: SAMD21-based boards (Arduino Zero, MKR1000, Adafruit Feather M0, Sparkfun SAMD21 Mini Breakout, etc), SAMD51-based boards (Adafruit Grand Central M4, SparkFun SAMD51 Thing Plus, etc).
 category=Timing
 url=http://www.arduino.cc/en/Reference/RTC
 architectures=samd