dt-binding: spi: add bindings for spi-ar934x

Add binding documentation for SPI controller in Qualcomm Atheros
AR934x/QCA95xx SoCs.
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
Link: https://lore.kernel.org/r/20200210034152.49063-3-gch981213@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/qca,ar934x-spi.yaml

+# SPDX-License-Identifier: GPL-2.0
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/spi/qca,ar934x-spi.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Qualcomm Atheros AR934x/QCA95xx SoC SPI controller
+
+maintainers:
+  - Chuanhong Guo <gch981213@gmail.com>
+
+allOf:
+  - $ref: spi-controller.yaml#
+
+properties:
+  compatible:
+    const: qca,ar934x-spi
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+  - clocks
+  - '#address-cells'
+  - '#size-cells'
+
+examples:
+  - |
+    #include <dt-bindings/clock/ath79-clk.h>
+    spi: spi@1f000000 {
+        compatible = "qca,ar934x-spi";
+        reg = <0x1f000000 0x1c>;
+        clocks = <&pll ATH79_CLK_AHB>;
+        #address-cells = <1>;
+        #size-cells = <0>;
+    };