overlay: Documentation for the overlay sugar syntax

There exists a syntactic sugar version of overlays which
make them simpler to write for the trivial case of a single target.

Document it in the device tree object internals.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>

Downloaded from https://marc.info/?l=devicetree&m=148111492925863&w=4

Change-Id: Iac4cb844f655516022926e7c18e7de3ae384e5e8