drivers: add driver for FXOS8700 3-axis accelerometer/magnetometer

drivers/Makefile.dep

drivers/Makefile.include

drivers/fxos8700/Makefile

drivers/fxos8700/fxos8700.c

drivers/fxos8700/fxos8700_saul.c

drivers/fxos8700/include/fxos8700_params.h

drivers/fxos8700/include/fxos8700_regs.h

drivers/include/fxos8700.h

sys/auto_init/auto_init.c

sys/auto_init/saul/auto_init_fxos8700.c

tests/driver_fxos8700/Makefile

tests/driver_fxos8700/README.md

tests/driver_fxos8700/main.c