dt-bindings: nvmem: add Broadcom's NVRAM

Broadcom's NVRAM structure contains device data and can be accessed
using I/O mapping.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20210330111241.19401-5-srinivas.kandagatla@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/nvmem/brcm,nvram.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/nvmem/brcm,nvram.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Broadcom's NVRAM
+
+description: |
+  Broadcom's NVRAM is a structure containing device specific environment
+  variables. It is used for storing device configuration, booting parameters
+  and calibration data.
+
+  NVRAM can be accessed on Broadcom BCM47xx MIPS and Northstar ARM Cortex-A9
+  devices usiong I/O mapped memory.
+
+maintainers:
+  - Rafał Miłecki <rafal@milecki.pl>
+
+allOf:
+  - $ref: "nvmem.yaml#"
+
+properties:
+  compatible:
+    const: brcm,nvram
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    nvram@1eff0000 {
+            compatible = "brcm,nvram";
+            reg = <0x1eff0000 0x10000>;
+    };