Update for use RTC ATMEGA4809 circuit embedded in Arduino Uno WiFi rev 2

[goodchip]

For review before pull-request

I've updated original TimeLib.h library for use with internal RTC clock of Arduino Uno WiFi rev 2 https://github.com/goodchip/Time

Examples here: https://github.com/goodchip/Time/tree/master/examples/TimeInternalRTC

Features:

• use internal RTC circuit for real clock in board (not based on millis() function)
• additional attach/detach interrupt function to run user function at adjustable freq. synchronized by RTC
• adjustable top syncho between RTC and real second when setTime is called
• 100% compatible with 328p code (run without RTC) if interrupt are not used

Current limitation:

• use only external quartz for the moment.

[goodchip]

Ideas / comments / suggestions are welcome ;)

[goodchip]

Major upgrades - very stable and usefull version

New! 2 interrupts features:

• InternalRTC.attachInterrupt: attach a IRQ user function called at the freq frequency
• InternalRTC.attachClockInterrupt: attach a IRQ user function called each new second

Great improvements:

• removal of the function setPrecision(), now useless
• now always maximal RTC cycles precision without cpu load