dt-bindings: mips: Add Luton

Luton SoC belongs to the same family as Ocelot.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>