dt-bindings: imx: Add pinctrl binding doc for i.MX8MP

Add binding doc for i.MX8MP pinctrl driver.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1579052348-32167-1-git-send-email-Anson.Huang@nxp.com


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