add native cpu doxygen documentation,

cpu/cc430/cc430-rtc.c

cpu/lpc2387/lpc2387-lpm.c

cpu/native/hwtimer_cpu.c

cpu/native/include/cpu.h

cpu/native/include/hwtimer_cpu.h

cpu/native/irq_cpu.c

cpu/native/lpm_cpu.c

cpu/native/native_cpu.c

cpu/native/rtc/posix-rtc.c

drivers/include/ltc4150_arch.h

drivers/include/rtc.h

drivers/include/sht11.h