Add atmega48p device support

Add basic support for ATmega168

Add support for ATtiny88

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