Merge pull request #4502 from haukepetersen/fix_doc_cc2538

boards/cc2538dk/board.c

boards/cc2538dk/include/periph_conf.h

cpu/cc2538/doc.txt

cpu/cc2538/include/cc2538.h

cpu/cc2538/include/cc2538_gpio.h

cpu/cc2538/include/cc2538_gptimer.h

cpu/cc2538/include/cc2538_ioc.h

cpu/cc2538/include/cc2538_rfcore.h

cpu/cc2538/include/cc2538_soc_adc.h

cpu/cc2538/include/cc2538_sys_ctrl.h

cpu/cc2538/include/cc2538_uart.h

cpu/cc2538/include/cpu_conf.h

cpu/cc2538/include/gptimer.h