dt-bindings: nvmem: Add bindings for rmem driver

Firmware/co-processors might use reserved memory areas in order to pass
data stemming from an nvmem device otherwise non accessible to Linux.
For example an EEPROM memory only physically accessible to firmware, or
data only accessible early at boot time.

Introduce the dt-bindings to nvmem's rmem.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20210129171430.11328-4-srinivas.kandagatla@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/nvmem/rmem.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/nvmem/rmem.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Reserved Memory Based nvmem Device
+
+maintainers:
+  - Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
+
+allOf:
+  - $ref: "nvmem.yaml#"
+
+properties:
+  compatible:
+    items:
+      - enum:
+          - raspberrypi,bootloader-config
+      - const: nvmem-rmem
+
+  no-map:
+    $ref: /schemas/types.yaml#/definitions/flag
+    description:
+      Avoid creating a virtual mapping of the region as part of the OS'
+      standard mapping of system memory.
+
+required:
+  - compatible
+  - no-map
+
+unevaluatedProperties: false
+
+examples:
+  - |
+        reserved-memory {
+                #address-cells = <1>;
+                #size-cells = <1>;
+
+                blconfig: nvram@10000000 {
+                        compatible = "raspberrypi,bootloader-config", "nvmem-rmem";
+                        #address-cells = <1>;
+                        #size-cells = <1>;
+                        reg = <0x10000000 0x1000>;
+                        no-map;
+                };
+        };
+
+...