Add support for ATmega2560

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>

Switch to a new design as outlined in #17

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

Added atmega1280.

Add support for atmega8

Signed-off-by: Filip <filip.skubacz.public@gmail.com>

Add support for atmega328p

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

Add interrupts and use cfg-if

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

Initial