Merge pull request #7129 from haukepetersen/opt_periph_sharetimerset

cpu/atmega_common/periph/timer.c

cpu/cc2538/periph/timer.c

cpu/cc26x0/periph/timer.c

cpu/cc430/periph/timer.c

cpu/ezr32wg/periph/timer.c

cpu/kinetis_common/include/periph_cpu.h

cpu/lm4f120/periph/timer.c

cpu/lpc11u34/periph/timer.c

cpu/lpc1768/periph/timer.c

cpu/lpc2387/periph/timer.c

cpu/mips_pic32_common/include/periph_cpu_common.h

cpu/msp430fxyz/periph/timer.c

cpu/native/include/periph_cpu.h

cpu/nrf5x_common/periph/timer.c

cpu/sam3/periph/timer.c

cpu/samd21/periph/timer.c

cpu/saml21/periph/timer.c

cpu/stm32_common/periph/timer.c

drivers/periph_common/timer.c