dt-bindings: add vendor prefix for "Endless Mobile, Inc."

Endless Mobile, Inc. started selling (small) computers in 2015.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>