cpu*: fix doxygen grouping

cpu/cc2538/periph/hwrng.c

cpu/cc26x0/include/cc26x0_i2c.h

cpu/cc26x0/include/cc26x0_ioc.h

cpu/cc430/cc430-adc.c

cpu/cc430/doc.txt

cpu/cc430/include/cc430-rtc.h

cpu/cc430/periph/rtc.c

cpu/cc430/periph/timer.c

cpu/cortexm_common/panic.c

cpu/efm32/doc.txt

cpu/kinetis/include/vectors_kinetis.h

cpu/kinetis/periph/hwrng.c

cpu/kinetis/periph/i2c.c

cpu/kinetis/periph/mcg.c

cpu/kinetis/periph/wdog.c

cpu/lpc2387/cpu.c

cpu/lpc2387/doc/lpc2387.doc

cpu/lpc2387/include/cpu.h

cpu/lpc2387/include/cpu_conf.h

cpu/lpc2387/lpc23xx-iap.c