dt-bindings: arm: bcm: document BCM4908 bindings

BCM4908 is a new family that includes BCM4906, BCM4908 and BCM49408.
It's mostly used in home routers and often replaces Northstar in vendors
portfolio.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

Documentation/devicetree/bindings/arm/bcm/brcm,bcm4908.yaml

+# SPDX-License-Identifier: GPL-2.0 OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/arm/bcm/brcm,bcm4908.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Broadcom BCM4908 device tree bindings
+
+description:
+  Broadcom BCM4906 / BCM4908 / BCM49408 Wi-Fi/network SoCs with Brahma CPUs.
+
+maintainers:
+  - Rafał Miłecki <rafal@milecki.pl>
+
+properties:
+  $nodename:
+    const: '/'
+  compatible:
+    oneOf:
+      - description: BCM4906 based boards
+        items:
+          - const: brcm,bcm4906
+          - const: brcm,bcm4908
+
+      - description: BCM4908 based boards
+        items:
+          - enum:
+              - asus,gt-ac5300
+          - const: brcm,bcm4908
+
+      - description: BCM49408 based boards
+        items:
+          - const: brcm,bcm49408
+          - const: brcm,bcm4908
+
+additionalProperties: true
+
+...