scripts: utils: add pinctrl migration script for nRF boards

This script can be used to automatically migrate the Devicetree files of
nRF-based boards using the old <signal>-pin properties to select
peripheral pins. The script will parse a board Devicetree file and will
first adjust that file by removing old pin-related properties replacing
them with pinctrl states.  A board-pinctrl.dtsi file will be generated
containing the configuration for all pinctrl states. Note that script
will also work on files that have been partially ported.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>