dt-bindings: zx296718-clk: add compatible for audio clock controller

It adds the compatible string for zx296718 audio clock controller.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>