cores: Fix ADCH and ADCL to use defines for RFA1

No current release provides these defines however they are fixed in
avr-libc trunk and as such we should use them if they are available.
Clean up outdated comment

This comment is no longer true for any of the listed platforms so its
best to adjust the wording to what might be relevant to users
cores: Add initial support for ATmega128RFR2

Add initial support for the ATmega128RFR2 which according to Atmel is a
drop in replacement for the ATmega128RFA1 however in real world testing
I have found this to not be entirely true. All of the changes (new
features) added to the R2 are not exposed with current simavr
peripherals so this file is identical to the A1 in simavr.
cores: RFA1/RFR2 don't have Port A and Port C

The ATmega128RFA1 and ATmega128RFR2 don't actually have Port A and Port
C as IO pins.