dt-bindings: document rockchip rk3399-evb board

Use "rockchip,rk3399-evb" compatible string for Rockchip RK3399
evaluation board.

Signed-off-by: Jianqun Xu <jay.xu@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>