ixp4xx_eth: Add devicetree bindings

This adds device tree bindings for the IXP46x PTP Timer, a companion
to the IXP4xx ethernet in newer platforms.

Cc: devicetree@vger.kernel.org
Cc: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/intel,ixp46x-ptp-timer.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+# Copyright 2018 Linaro Ltd.
+%YAML 1.2
+---
+$id: "http://devicetree.org/schemas/net/intel,ixp46x-ptp-timer.yaml#"
+$schema: "http://devicetree.org/meta-schemas/core.yaml#"
+
+title: Intel IXP46x PTP Timer (TSYNC)
+
+maintainers:
+  - Linus Walleij <linus.walleij@linaro.org>
+
+description: |
+  The Intel IXP46x PTP timer is known in the manual as IEEE1588 Hardware
+  Assist and Time Synchronization Hardware Assist TSYNC provides a PTP
+  timer. It exists in the Intel IXP45x and IXP46x XScale SoCs.
+
+properties:
+  compatible:
+    const: intel,ixp46x-ptp-timer
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    items:
+      - description: Interrupt to trigger master mode snapshot from the
+          PRP timer, usually a GPIO interrupt.
+      - description: Interrupt to trigger slave mode snapshot from the
+          PRP timer, usually a GPIO interrupt.
+
+  interrupt-names:
+    items:
+      - const: master
+      - const: slave
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - interrupt-names
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/interrupt-controller/irq.h>
+    ptp-timer@c8010000 {
+        compatible = "intel,ixp46x-ptp-timer";
+        reg = <0xc8010000 0x1000>;
+        interrupt-parent = <&gpio0>;
+        interrupts = <8 IRQ_TYPE_EDGE_FALLING>, <7 IRQ_TYPE_EDGE_FALLING>;
+        interrupt-names = "master", "slave";
+    };