cpu/sam0: renamed sam21_common to sam0_common

cpu/sam0_common/Makefile

cpu/sam0_common/Makefile.include

cpu/sam0_common/include/cmsis/samd21/README.md

cpu/sam0_common/include/cmsis/samd21/include/component/ac.h

cpu/sam0_common/include/cmsis/samd21/include/component/adc.h

cpu/sam0_common/include/cmsis/samd21/include/component/dac.h

...sam0_common/include/cmsis/samd21/include/component/dmac.h

cpu/sam0_common/include/cmsis/samd21/include/component/dsu.h

cpu/sam0_common/include/cmsis/samd21/include/component/eic.h

...am0_common/include/cmsis/samd21/include/component/evsys.h

...sam0_common/include/cmsis/samd21/include/component/gclk.h

..._common/include/cmsis/samd21/include/component/hmatrixb.h

cpu/sam0_common/include/cmsis/samd21/include/component/i2s.h

cpu/sam0_common/include/cmsis/samd21/include/component/mtb.h

...0_common/include/cmsis/samd21/include/component/nvmctrl.h

cpu/sam0_common/include/cmsis/samd21/include/component/pac.h

cpu/sam0_common/include/cmsis/samd21/include/component/pm.h

...sam0_common/include/cmsis/samd21/include/component/port.h

cpu/sam0_common/include/cmsis/samd21/include/component/rtc.h

...m0_common/include/cmsis/samd21/include/component/sercom.h