boards/common/nrf52: improve default clock config
boards/common/nrf52: improve default clock config

- All nRF52 timers support 32 bit mode, so use that
- All nRF52 timers support at least 4 channels, the timers NRF_TIMER3
  and NRF_TIMER4 even support 6 channels.
- Add a warning that `TIMER_DEV(1)` is used by the IEEE 802.15.4 driver

RIOT-OS/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT