dt-bindings: mfd: ti,j721e-system-controller.yaml: Add J721e system controller

Add DT binding schema for J721e system controller.
Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tero Kristo <t-kristo@ti.com>

Documentation/devicetree/bindings/mfd/ti,j721e-system-controller.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+# Copyright (C) 2020 Texas Instruments Incorporated - http://www.ti.com/
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mfd/ti,j721e-system-controller.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: TI J721e System Controller Registers R/W Device Tree Bindings
+
+description: |
+  This represents the Control Module registers (CTRL_MMR0) on the SoC.
+  System controller node represents a register region containing a set
+  of miscellaneous registers. The registers are not cohesive enough to
+  represent as any specific type of device. The typical use-case is
+  for some other node's driver, or platform-specific code, to acquire
+  a reference to the syscon node (e.g. by phandle, node path, or
+  search using a specific compatible value), interrogate the node (or
+  associated OS driver) to determine the location of the registers,
+  and access the registers directly.
+
+maintainers:
+  - Kishon Vijay Abraham I <kishon@ti.com>
+  - Roger Quadros <rogerq@ti.com
+
+properties:
+  compatible:
+    anyOf:
+      - items:
+        - enum:
+           - ti,j721e-system-controller
+        - const: syscon
+        - const: simple-mfd
+
+  "#address-cells":
+    const: 1
+
+  "#size-cells":
+    const: 1
+
+  ranges: true
+
+# Optional children
+
+  "^serdes-ln-ctrl@[0-9a-f]+$":
+    type: object
+    description: |
+      This is the SERDES lane control mux. It should follow the bindings
+      specified in
+      Documentation/devicetree/bindings/mux/reg-mux.txt
+
+required:
+  - compatible
+  - reg
+  - "#address-cells"
+  - "#size-cells"
+  - ranges
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    scm_conf: scm-conf@100000 {
+        compatible = "ti,j721e-system-controller", "syscon", "simple-mfd";
+        reg = <0x00100000 0x1c000>;
+        #address-cells = <1>;
+        #size-cells = <1>;
+        ranges;
+
+        serdes_ln_ctrl: serdes-ln-ctrl@4080 {
+            compatible = "mmio-mux";
+            reg = <0x00004080 0x50>;
+        };
+    };
+...