dt-bindings: add rk3288-based Asus Tinker board

Tinker board is a credit card size develop board designed by Asus.
Powered by RK3288.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Eddie Cai <eddie.cai.linux@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>