cpu/native: fix doxygen grouping

cpu/native/async_read.c

cpu/native/include/async_read.h

cpu/native/include/cpu.h

cpu/native/include/cpu_conf.h

cpu/native/include/native_internal.h

cpu/native/include/netdev_tap.h

cpu/native/include/netdev_tap_params.h

cpu/native/include/periph_conf.h

cpu/native/include/periph_cpu.h

cpu/native/include/tty_uart.h

cpu/native/irq_cpu.c

cpu/native/native_cpu.c

cpu/native/netdev_tap/netdev_tap.c

cpu/native/osx-libc-extra/malloc.h

cpu/native/panic.c

cpu/native/periph/gpio.c

cpu/native/periph/hwrng.c

cpu/native/periph/pm.c

cpu/native/periph/qdec.c

cpu/native/periph/timer.c