dt-bindings: add bindings doc for ZTE pinctrl

It adds device tree bindings for ZTE pin controller found on ZX2967xx
family SoCs.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>