CPU: fix include header guards

cpu/atmega2560/include/cpu_conf.h

cpu/atmega_common/include/cpu.h

cpu/cc2538/include/cpu_conf.h

cpu/cortexm_common/include/cmsis_armcc.h

cpu/cortexm_common/include/cmsis_armcc_V6.h

cpu/cortexm_common/include/cmsis_gcc.h

cpu/ezr32wg/include/ezr32wg330f256r60.h

cpu/k64f/include/cpu_conf.h

cpu/kinetis_common/include/mcg.h

cpu/kw2x/include/cpu_conf.h

cpu/lpc11u34/include/LPC11Uxx.h

cpu/lpc11u34/include/cpu_conf.h

cpu/lpc1768/include/LPC17xx.h

cpu/lpc1768/include/cpu_conf.h

cpu/lpc2387/include/cpu.h

cpu/lpc2387/include/i2c.h

cpu/lpc2387/include/lpc2387.h

cpu/saml21/include/cpu_conf.h