devicetree: bindings: Add vendor prefix for Oki

Already in use for "oki,ml86v7667".

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Rob Herring <robh@kernel.org>