pkg/gecko_sdk: update to version 2.6
pkg/gecko_sdk: update to version 2.6

jasonatran/RIOT,josephnoir/RIOT,kaspar030/RIOT,toonst/RIOT,aeneby/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,kYc0o/RIOT,authmillenon/RIOT,jasonatran/RIOT,ant9000/RIOT,aeneby/RIOT,OlegHahm/RIOT,x3ro/RIOT,authmillenon/RIOT,miri64/RIOT,RIOT-OS/RIOT,basilfx/RIOT,yogo1212/RIOT,ant9000/RIOT,jasonatran/RIOT,authmillenon/RIOT,x3ro/RIOT,miri64/RIOT,mtausig/RIOT,yogo1212/RIOT,mtausig/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,x3ro/RIOT,kaspar030/RIOT,ant9000/RIOT,josephnoir/RIOT,kaspar030/RIOT,miri64/RIOT,OlegHahm/RIOT,kYc0o/RIOT,kaspar030/RIOT,josephnoir/RIOT,yogo1212/RIOT,OTAkeys/RIOT,mtausig/RIOT,kYc0o/RIOT,authmillenon/RIOT,authmillenon/RIOT,ant9000/RIOT,toonst/RIOT,OTAkeys/RIOT,toonst/RIOT,kaspar030/RIOT,mtausig/RIOT,basilfx/RIOT,yogo1212/RIOT,OTAkeys/RIOT,smlng/RIOT,miri64/RIOT,OlegHahm/RIOT,miri64/RIOT,OTAkeys/RIOT,mtausig/RIOT,smlng/RIOT,basilfx/RIOT,kYc0o/RIOT,smlng/RIOT,aeneby/RIOT,OlegHahm/RIOT,kYc0o/RIOT,basilfx/RIOT,smlng/RIOT,aeneby/RIOT,ant9000/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,jasonatran/RIOT,toonst/RIOT,authmillenon/RIOT,josephnoir/RIOT,smlng/RIOT,josephnoir/RIOT,aeneby/RIOT,OlegHahm/RIOT,x3ro/RIOT,toonst/RIOT,basilfx/RIOT,x3ro/RIOT,RIOT-OS/RIOT,jasonatran/RIOT