cpu/nrf5x: unified clock initialization code

boards/airfy-beacon/include/periph_conf.h

boards/calliope-mini/include/periph_conf.h

boards/microbit/include/periph_conf.h

boards/nrf51dongle/include/periph_conf.h

boards/nrf52840dk/include/periph_conf.h

boards/nrf52dk/include/periph_conf.h

boards/nrf6310/include/periph_conf.h

boards/pca10000/include/periph_conf.h

boards/pca10005/include/periph_conf.h

boards/yunjia-nrf51822/include/periph_conf.h

cpu/nrf51/cpu.c

cpu/nrf52/cpu.c

cpu/nrf5x_common/Makefile

cpu/nrf5x_common/Makefile.include

cpu/nrf5x_common/clock.c

cpu/nrf5x_common/include/nrf_clock.h