clk: rockchip: Add dt-binding header for rk3308

Add the dt-bindings header for the rk3308, that gets shared between
the clock controller and the clock references in the dts.

Signed-off-by: Finley Xiao <finley.xiao@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>