samples: boards: nrf: dynamic_pinctrl: initial version

Add a sample that shows how dynamic pin control can be useful to remap a
peripheral to a different set of pins.

NOTE: default pin configuration is provided for the nRF52840-DK board.
Such configuration will be removed once all nRF-based boards are ported
to pinctrl.

Thanks to Francesco for documentation suggestions.

Co-authored-by: Francesco D. Servidio <francesco.servidio@nordicsemi.no>
Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>