cpu: atmega2560: Initial import

cpu/atmega2560/Makefile

cpu/atmega2560/Makefile.include

cpu/atmega2560/cpu.c

cpu/atmega2560/doc.txt

cpu/atmega2560/hwtimer_arch.c

cpu/atmega2560/include/cpu-conf.h

cpu/atmega2560/include/hwtimer_cpu.h

cpu/atmega2560/io_arch.c

cpu/atmega2560/lpm_arch.c

cpu/atmega2560/periph/Makefile

cpu/atmega2560/periph/gpio.c

cpu/atmega2560/periph/timer.c

cpu/atmega2560/periph/uart.c

cpu/atmega2560/reboot_arch.c

cpu/atmega2560/startup.c