The `channels` member should not be set to the number of hardware
channels *n*, but to *n* - 1 instead. The last channel is implicitly
used in `timer_read()`. Hence out of *n* hardware channels, only *n* - 1
are available to the application.
This fixes a bug introduced by 4d02e15247
which incorrectly set the channel number to *n* rather than to
*n* - 1.
- 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
