gen_edt: use workspace dir as base for relative paths in comments

This commit allows comments to reference files with paths that are relative
to the Zephyr workspace root. This is done by adding a new argument
'--workspace-dir' to the 'gen_edt.py' script, which is passed to the
'EDT' and 'DT' classes and used instead of the current working directory.

The workspace directory is set to WEST_TOPDIR if West is in use,
otherwise it is set to the parent directory of ZEPHYR_BASE so that
Zephyr files have a 'zephyr/' prefix.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>