cpu/cc2538: cleanup periph timer implementation
cpu/cc2538: cleanup periph timer implementation

    Refine periph timer implementation to use vendor header defines
    where possible, remove unnecessary structs and general cleanup.

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