Add support for ATmega164pa

Adds the ATDF file provided from Microchip, and makes changes analog to
290613454fbdc5e4ac98e53deccaf74dafc88963.
This has been tested on real hardware.

Signed-off-by: Tronje Krabbe <tkrabbe@jusst.de>
Signed-off-by: Oskar Munz <omunz@jusst.de>