dt-bindings: add documentation of rk3668 clock controller

Add the devicetree binding for the cru on the rk3368 which quite similar
structured as previous clock controllers.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>