dt-bindings: imx: Add pinctrl binding doc for imx8mm

* Freescale IMX8MM IOMUX Controller

Please refer to fsl,imx-pinctrl.txt and pinctrl-bindings.txt in this directory

for common binding part and usage.

Required properties:

- compatible: "fsl,imx8mm-iomuxc"

- reg: should contain the base physical address and size of the iomuxc

  registers.

Required properties in sub-nodes:

- fsl,pins: each entry consists of 6 integers and represents the mux and config

  setting for one pin.  The first 5 integers <mux_reg conf_reg input_reg mux_val

  input_val> are specified using a PIN_FUNC_ID macro, which can be found in

  <dt-bindings/pinctrl/imx8mm-pinfunc.h>. The last integer CONFIG is

  the pad setting value like pull-up on this pin.  Please refer to i.MX8M Mini

  Reference Manual for detailed CONFIG settings.

Examples:

&uart1 {

       pinctrl-names = "default";

       pinctrl-0 = <&pinctrl_uart1>;

};

iomuxc: pinctrl@30330000 {

        compatible = "fsl,imx8mm-iomuxc";

        reg = <0x0 0x30330000 0x0 0x10000>;

        pinctrl_uart1: uart1grp {

                fsl,pins = <

                        MX8MM_IOMUXC_UART1_RXD_UART1_DCE_RX             0x140

                        MX8MM_IOMUXC_UART1_TXD_UART1_DCE_TX             0x140

                >;

        };

};