cpu/esp32: define BACKUP_RAM attribute
cpu/esp32: define BACKUP_RAM attribute

kaspar030/RIOT,OTAkeys/RIOT,basilfx/RIOT,jasonatran/RIOT,ant9000/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kYc0o/RIOT,kaspar030/RIOT,yogo1212/RIOT,OTAkeys/RIOT,basilfx/RIOT,basilfx/RIOT,kaspar030/RIOT,ant9000/RIOT,OlegHahm/RIOT,authmillenon/RIOT,OlegHahm/RIOT,kYc0o/RIOT,jasonatran/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,kYc0o/RIOT,basilfx/RIOT,kaspar030/RIOT,miri64/RIOT,ant9000/RIOT,kaspar030/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,miri64/RIOT,basilfx/RIOT,authmillenon/RIOT,yogo1212/RIOT,miri64/RIOT,yogo1212/RIOT,kYc0o/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,miri64/RIOT,miri64/RIOT,ant9000/RIOT,yogo1212/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,kYc0o/RIOT,jasonatran/RIOT,authmillenon/RIOT,yogo1212/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,ant9000/RIOT,authmillenon/RIOT