dt-bindings: Document rk3399 Gru/Kevin

Gru is a base dev board for a family of devices, including Kevin. Both
utilize Rockchip RK3399, and they share much of their design.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
[added Samsung Chromebook Plus hint]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>