dt-bindings: vendors: add Elimo Engineering vendor prefix

Add elimo as vendor prefix for dt bindings, since we are adding a dtsi
for a SoM and a dts for an SBC

Signed-off-by: Matteo Scordino <matteo.scordino@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20201030234325.5865-2-matteo.scordino@gmail.com