dt-bindings: add documentation for rk3288 cru

This adds the dt-binding documentation for the clock and reset unit found on
Rockchip rk3288 SoCs.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-By: Max Schwarz <max.schwarz@online.de>
Tested-By: Max Schwarz <max.schwarz@online.de>
Signed-off-by: Mike Turquette <mturquette@linaro.org>