atmega_common: make rom and ram length definition mandatory
atmega_common: make rom and ram length definition mandatory

Now all atmega cpu define them so use them by default.

lazytech-org/RIOT,smlng/RIOT,miri64/RIOT,authmillenon/RIOT,jasonatran/RIOT,BytesGalore/RIOT,avmelnikoff/RIOT,ant9000/RIOT,RIOT-OS/RIOT,A-Paul/RIOT,OTAkeys/RIOT,toonst/RIOT,kYc0o/RIOT,kbumsik/RIOT,OlegHahm/RIOT,avmelnikoff/RIOT,avmelnikoff/RIOT,gebart/RIOT,BytesGalore/RIOT,yogo1212/RIOT,ant9000/RIOT,mtausig/RIOT,mtausig/RIOT,kbumsik/RIOT,A-Paul/RIOT,aeneby/RIOT,lazytech-org/RIOT,A-Paul/RIOT,cladmi/RIOT,mfrey/RIOT,cladmi/RIOT,x3ro/RIOT,yogo1212/RIOT,rfuentess/RIOT,x3ro/RIOT,aeneby/RIOT,lazytech-org/RIOT,biboc/RIOT,kaspar030/RIOT,lazytech-org/RIOT,avmelnikoff/RIOT,yogo1212/RIOT,rfuentess/RIOT,aeneby/RIOT,toonst/RIOT,josephnoir/RIOT,yogo1212/RIOT,kYc0o/RIOT,cladmi/RIOT,aeneby/RIOT,josephnoir/RIOT,yogo1212/RIOT,biboc/RIOT,authmillenon/RIOT,biboc/RIOT,avmelnikoff/RIOT,authmillenon/RIOT,authmillenon/RIOT,gebart/RIOT,x3ro/RIOT,jasonatran/RIOT,ant9000/RIOT,miri64/RIOT,rfuentess/RIOT,miri64/RIOT,x3ro/RIOT,kYc0o/RIOT,A-Paul/RIOT,RIOT-OS/RIOT,cladmi/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,gebart/RIOT,mfrey/RIOT,jasonatran/RIOT,A-Paul/RIOT,RIOT-OS/RIOT,toonst/RIOT,smlng/RIOT,jasonatran/RIOT,x3ro/RIOT,kaspar030/RIOT,miri64/RIOT,smlng/RIOT,kaspar030/RIOT,gebart/RIOT,kYc0o/RIOT,kbumsik/RIOT,yogo1212/RIOT,basilfx/RIOT,kaspar030/RIOT,kbumsik/RIOT,RIOT-OS/RIOT,aeneby/RIOT,miri64/RIOT,BytesGalore/RIOT,basilfx/RIOT,OTAkeys/RIOT,basilfx/RIOT,toonst/RIOT,OlegHahm/RIOT,kbumsik/RIOT,BytesGalore/RIOT,josephnoir/RIOT,mfrey/RIOT,OTAkeys/RIOT,BytesGalore/RIOT,basilfx/RIOT,kYc0o/RIOT,mtausig/RIOT,lazytech-org/RIOT,OlegHahm/RIOT,toonst/RIOT,authmillenon/RIOT,OlegHahm/RIOT,josephnoir/RIOT,jasonatran/RIOT,mfrey/RIOT,gebart/RIOT,smlng/RIOT,mtausig/RIOT,mfrey/RIOT,smlng/RIOT,OlegHahm/RIOT,josephnoir/RIOT,ant9000/RIOT,ant9000/RIOT,biboc/RIOT,rfuentess/RIOT,rfuentess/RIOT,basilfx/RIOT,biboc/RIOT,OTAkeys/RIOT,cladmi/RIOT,authmillenon/RIOT,mtausig/RIOT