dt-bindings: Add vendor prefix for Rakuten Kobo, Inc.

Rakuten Kobo, Inc. (formerly Kobo, Inc.) is a company that sells e-book
readers and related products. More information is available at:

- https://en.wikipedia.org/wiki/Kobo_Inc.
- https://www.kobo.com/



Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>