dt-bindings: mtd: add basic bindings for UBI

Add basic bindings for UBI devices and volumes.
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Richard Weinberger <richard@nod.at>

Documentation/devicetree/bindings/mtd/partitions/linux,ubi.yaml

+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mtd/partitions/linux,ubi.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Unsorted Block Images
+
+description: |
+  UBI ("Unsorted Block Images") is a volume management system for raw
+  flash devices which manages multiple logical volumes on a single
+  physical flash device and spreads the I/O load (i.e wear-leveling)
+  across the whole flash chip.
+
+maintainers:
+  - Daniel Golle <daniel@makrotopia.org>
+
+allOf:
+  - $ref: partition.yaml#
+
+properties:
+  compatible:
+    const: linux,ubi
+
+  volumes:
+    type: object
+    description: UBI Volumes
+
+    patternProperties:
+      "^ubi-volume-.*$":
+        $ref: /schemas/mtd/partitions/ubi-volume.yaml#
+
+    unevaluatedProperties: false
+
+required:
+  - compatible
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    partitions {
+        compatible = "fixed-partitions";
+        #address-cells = <1>;
+        #size-cells = <1>;
+
+        partition@0 {
+            reg = <0x0 0x100000>;
+            label = "bootloader";
+            read-only;
+        };
+
+        partition@100000 {
+            reg = <0x100000 0x1ff00000>;
+            label = "ubi";
+            compatible = "linux,ubi";
+
+            volumes {
+                ubi-volume-caldata {
+                    volid = <2>;
+                    volname = "rf";
+                };
+            };
+        };
+    };

Documentation/devicetree/bindings/mtd/partitions/ubi-volume.yaml

+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mtd/partitions/ubi-volume.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: UBI volume
+
+description: |
+  This binding describes a single UBI volume. Volumes can be matches either
+  by their ID or their name, or both.
+
+maintainers:
+  - Daniel Golle <daniel@makrotopia.org>
+
+properties:
+  volid:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description:
+      Match UBI volume ID
+
+  volname:
+    $ref: /schemas/types.yaml#/definitions/string
+    description:
+      Match UBI volume ID
+
+anyOf:
+  - required:
+      - volid
+
+  - required:
+      - volname
+
+# This is a generic file other binding inherit from and extend
+additionalProperties: true