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>

Documentation/devicetree/bindings/pinctrl/fsl,imx8mp-pinctrl.yaml

@@ -0,0 +1,69 @@
+# SPDX-License-Identifier: GPL-2.0
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/pinctrl/fsl,imx8mp-pinctrl.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Freescale IMX8MP IOMUX Controller
+
+maintainers:
+  - Anson Huang <Anson.Huang@nxp.com>
+
+description:
+  Please refer to fsl,imx-pinctrl.txt and pinctrl-bindings.txt in this directory
+  for common binding part and usage.
+
+properties:
+  compatible:
+    const: fsl,imx8mp-iomuxc
+
+  reg:
+    maxItems: 1
+
+# Client device subnode's properties
+patternProperties:
+  'grp$':
+    type: object
+    description:
+      Pinctrl node's client devices use subnodes for desired pin configuration.
+      Client device subnodes use below standard properties.
+
+    properties:
+      fsl,pins:
+        allOf:
+          - $ref: /schemas/types.yaml#/definitions/uint32-array
+        description:
+          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 <arch/arm64/boot/dts/freescale/imx8mp-pinfunc.h>. The last
+          integer CONFIG is the pad setting value like pull-up on this pin. Please
+          refer to i.MX8M Plus Reference Manual for detailed CONFIG settings.
+
+    required:
+      - fsl,pins
+
+    additionalProperties: false
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  # Pinmux controller node
+  - |
+    iomuxc: pinctrl@30330000 {
+        compatible = "fsl,imx8mp-iomuxc";
+        reg = <0x30330000 0x10000>;
+
+        pinctrl_uart2: uart2grp {
+            fsl,pins = <
+                0x228 0x488 0x5F0 0x0 0x6	0x49
+                0x228 0x488 0x000 0x0 0x0	0x49
+            >;
+        };
+    };
+
+...