dt: bindings: add documentation for zx2967 family i2c controller

This patch adds dt-binding documentation for zx2967 family
i2c controller.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>