dt-bindings: arm: rockchip: Add Kobol Helios64

Document the new board by Kobol introduced recently in
rockchip/rk3399-kobol-helios64.dts.

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201102150658.167161-1-uwe@kleine-koenig.org


Signed-off-by: Heiko Stuebner <heiko@sntech.de>