docs: dt: remove stale property-units.txt, link to current schema

property-units.txt is not maintained anymore. New suffixes are now
supported that have not been added here.

To avoid people incorrectly think a given suffix is not in the list remove
the file entirely and point to the authoritative place where the
recommended suffixes are maintained.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Link: https://lore.kernel.org/r/20210512152311.7399-1-luca@lucaceresoli.net


Signed-off-by: Rob Herring <robh@kernel.org>