Add patches for ADC peripherals

Add a patch to fixup a few registers in the ADC peripheral
for ATmega328P, ATmega32U4, ATmega1280, and ATmega64.

Add basic support for ATmega64

Pull in the ATDF file from Microchip and add all the necessary plumbing
around the code-base to make it compile.  Not tested against real
hardware and no device-specific patches are included yet.

Signed-off-by: Rahix <rahix@rahix.de>