dt-bindings: imx: Correct pinfunc head file path for i.MX8MM

The i.MX8MM pinfunc head file is located in DT folder, correct it.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>