drivers: adc: nrfx_saadc: Add support for SAADC internal sampling timer

The SAMPLERATE register can be used as a local timer instead
of triggering individual SAMPLE tasks. When SAMPLERATE.MODE is set
to Timers, it is sufficient to trigger SAMPLE task only once in order
to start the SAADC and triggering the STOP task will stop sampling.
The SAMPLERATE.CC field controls the sample rate.

The SAMPLERATE timer should not be combined with SCAN mode and
only one channel should be enabled when using the internal timer.

Signed-off-by: Jakub Zymelka <jakub.zymelka@nordicsemi.no>