dt-bindings: pinctrl: Add binding for ZynqMP pinctrl driver

Adding documentation and dt-bindings file which contains MIO pin
configuration defines for Xilinx ZynqMP pinctrl driver.

Signed-off-by: Sai Krishna Potthuri <lakshmi.sai.krishna.potthuri@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1618485193-5403-3-git-send-email-lakshmi.sai.krishna.potthuri@xilinx.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>