dt-bindings: reset: document Broadcom's BCM4908 PCIe reset binding

BCM4908 was built using older PCIe hardware block that requires using
external reset block controlling PERST# signals.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

Documentation/devicetree/bindings/reset/brcm,bcm4908-misc-pcie-reset.yaml

+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/reset/brcm,bcm4908-misc-pcie-reset.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Broadcom MISC block PCIe reset controller
+
+description: This document describes reset controller handling PCIe PERST#
+  signals. On BCM4908 it's a part of the MISC block.
+
+maintainers:
+  - Rafał Miłecki <rafal@milecki.pl>
+
+properties:
+  compatible:
+    const: brcm,bcm4908-misc-pcie-reset
+
+  reg:
+    maxItems: 1
+
+  "#reset-cells":
+    description: PCIe core id
+    const: 1
+
+required:
+  - compatible
+  - reg
+  - "#reset-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    reset-controller@ff802644 {
+        compatible = "brcm,bcm4908-misc-pcie-reset";
+        reg = <0xff802644 0x04>;
+        #reset-cells = <1>;
+    };