This function is supposed to be used for putting the CPU into
sleep mode for short amounts of time (e.g. in typical polling
loops used in periph drivers).
- added vendor header files for nrf52840
- added config for nrf52840
- added additional isr vectors for the nrf52840
- enabled port 1 in GPIO driver
- adapted periph drivers to cope with GPIO driver changes
- adapted the GPIO config of the I2C driver for the nrf51