overlay: Add syntactic sugar version of overlays

For simple overlays that use a single target there exists a
simpler syntax version.

&foo { }; generates an overlay with a single target at foo.

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

Downloaded from https://marc.info/?l=devicetree&m=148111493325866&w=4

Change-Id: I7bf2ff20133ee37888e8dc6031b5539b0d6168e7