dt-bindings: reserved-memory: Support MTD/block device

Add bindings to allow MTD/block devices to be used in reserved-memory
regions using the "phram" (MTD in PHysical RAM) driver.

This allows things like partitioning to be specified via the existing
devicetree bindings.
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220412135302.1682890-3-vincent.whitchurch@axis.com

Documentation/devicetree/bindings/reserved-memory/phram.yaml

+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/reserved-memory/phram.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: MTD/block device in RAM
+
+description: |
+  Specifies that the reserved memory region can be used as an MTD or block
+  device.
+
+  The "phram" node is named after the "MTD in PHysical RAM" driver which
+  provides an implementation of this functionality in Linux.
+
+maintainers:
+  - Vincent Whitchurch <vincent.whitchurch@axis.com>
+
+allOf:
+  - $ref: "reserved-memory.yaml"
+  - $ref: "/schemas/mtd/mtd.yaml"
+
+properties:
+  compatible:
+    const: phram
+
+  reg:
+    description: region of memory that can be used as an MTD/block device
+
+required:
+  - compatible
+  - reg
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    reserved-memory {
+        #address-cells = <1>;
+        #size-cells = <1>;
+
+        phram: flash@12340000 {
+            compatible = "phram";
+            label = "rootfs";
+            reg = <0x12340000 0x00800000>;
+        };
+    };