cpu/sam21_common: eliminate trailing white space in CMSIS include files

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

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

...sam21_common/include/cmsis/samr21/include/instance/gclk.h

...21_common/include/cmsis/samr21/include/instance/nvmctrl.h

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

...m21_common/include/cmsis/samr21/include/instance/rfctrl.h

...1_common/include/cmsis/samr21/include/instance/sbmatrix.h

...21_common/include/cmsis/samr21/include/instance/sercom0.h

...21_common/include/cmsis/samr21/include/instance/sercom1.h

...21_common/include/cmsis/samr21/include/instance/sercom2.h

...21_common/include/cmsis/samr21/include/instance/sercom3.h

...21_common/include/cmsis/samr21/include/instance/sercom4.h

...21_common/include/cmsis/samr21/include/instance/sercom5.h

...21_common/include/cmsis/samr21/include/instance/sysctrl.h

cpu/sam21_common/include/cmsis/samr21/include/instance/tc3.h

cpu/sam21_common/include/cmsis/samr21/include/instance/tc4.h

cpu/sam21_common/include/cmsis/samr21/include/instance/tc5.h

...sam21_common/include/cmsis/samr21/include/instance/tcc0.h

...sam21_common/include/cmsis/samr21/include/instance/tcc1.h

...sam21_common/include/cmsis/samr21/include/instance/tcc2.h