The `i2c_read_bytes()` and `i2c_write_bytes()` function return the
number of bytes written / read, instead of `0` as the API contract
says. This fixes the issue.
- 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