This refactors gen-device-avr to output two different formats: one for
all the existing AVR chips (that don't really have the concept of a
peripheral, just a bunch of registers), and one for all the new chips
like the ATtiny1616 (tinyAVR 1-series and 2-series) that have
peripherals like the Cortex-M chips with type structs and instances.
I checked the generated code for all the AVR chips we have support for
(atmega1280, atmega1284p, atmega2560, atmega328p, atmega32u4, attiny85)
and while the generated Go code did change, it looks safe to me.
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