Merge pull request #5743 from keestux/refactor-samd21

boards/samr21-xpro/Makefile.include

boards/samr21-xpro/include/periph_conf.h

cpu/sam21_common/Makefile.include

cpu/sam21_common/include/cmsis/samr21/README.md

cpu/sam21_common/include/cmsis/samr21/include/component/ac.h

...sam21_common/include/cmsis/samr21/include/component/adc.h

...sam21_common/include/cmsis/samr21/include/component/dac.h

...am21_common/include/cmsis/samr21/include/component/dmac.h

...sam21_common/include/cmsis/samr21/include/component/dsu.h

...sam21_common/include/cmsis/samr21/include/component/eic.h

...m21_common/include/cmsis/samr21/include/component/evsys.h

...am21_common/include/cmsis/samr21/include/component/gclk.h

..._common/include/cmsis/samr21/include/component/hmatrixb.h

...sam21_common/include/cmsis/samr21/include/component/i2s.h

...sam21_common/include/cmsis/samr21/include/component/mtb.h

...1_common/include/cmsis/samr21/include/component/nvmctrl.h

...sam21_common/include/cmsis/samr21/include/component/pac.h

cpu/sam21_common/include/cmsis/samr21/include/component/pm.h

...am21_common/include/cmsis/samr21/include/component/port.h

...21_common/include/cmsis/samr21/include/component/rfctrl.h