drivers/mma7760: Add initial support

drivers/Makefile.dep

drivers/Makefile.include

drivers/include/mma7660.h

drivers/mma7660/Makefile

drivers/mma7660/include/mma7660_params.h

drivers/mma7660/include/mma7660_reg.h

drivers/mma7660/mma7660.c

drivers/mma7660/mma7660_saul.c

sys/auto_init/auto_init.c

sys/auto_init/saul/auto_init_mma7660.c

tests/driver_mma7660/Makefile

tests/driver_mma7660/README.md

tests/driver_mma7660/main.c