cpus: make use of cortexm_isr_end()

cpu/cc2538/periph/gpio.c

cpu/cc2538/periph/i2c.c

cpu/cc2538/periph/spi.c

cpu/cc2538/periph/timer.c

cpu/cc2538/periph/uart.c

cpu/cc2538/radio/cc2538_rf_internal.c

cpu/cc26x0/periph/gpio.c

cpu/cc26x0/periph/timer.c

cpu/cc26x0/periph/uart.c

cpu/ezr32wg/periph/gpio.c

cpu/ezr32wg/periph/timer.c

cpu/ezr32wg/periph/uart.c

cpu/kinetis_common/periph/gpio.c

cpu/kinetis_common/periph/rtt.c

cpu/kinetis_common/periph/spi.c

cpu/kinetis_common/periph/timer.c

cpu/kinetis_common/periph/uart.c

cpu/lm4f120/periph/timer.c

cpu/lm4f120/periph/uart.c

cpu/lpc11u34/periph/gpio.c