dt-bindings: Add LM81 and DS1780 as trivial devices

The LM81 and DS1780 are close relatives of the ADM9240 and already
supported by the same driver. Document them as trivial devices.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Link: https://lore.kernel.org/r/20200923051637.30464-1-chris.packham@alliedtelesis.co.nz


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