spi: dt-bindings: Add binding document of Amlogic SPISG controller

The SPISG is a new communication oriented SPI controller of Amlogic, which
supports PIO, block DMA and scatter-gather DMA three operation modes.

Signed-off-by: Sunny Luo <sunny.luo@amlogic.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Xianwei Zhao <xianwei.zhao@amlogic.com>
Link: https://patch.msgid.link/20250718-spisg-v5-1-b8f0f1eb93a2@amlogic.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/amlogic,a4-spisg.yaml

@@ -0,0 +1,59 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+# Copyright (C) 2025 Amlogic, Inc. All rights reserved
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/spi/amlogic,a4-spisg.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Amlogic SPI Scatter-Gather Controller
+
+maintainers:
+  - Xianwei Zhao <xianwei.zhao@amlogic.com>
+  - Sunny Luo <sunny.luo@amlogic.com>
+
+allOf:
+  - $ref: spi-controller.yaml#
+
+properties:
+  compatible:
+    const: amlogic,a4-spisg
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    maxItems: 1
+
+  clocks:
+    maxItems: 2
+
+  clock-names:
+    items:
+      - const: core
+      - const: pclk
+
+  resets:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - clocks
+  - clock-names
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+    spi@50000 {
+        compatible = "amlogic,a4-spisg";
+        reg = <0x50000 0x38>;
+        interrupts = <GIC_SPI 183 IRQ_TYPE_LEVEL_HIGH>;
+        clocks = <&clkc 37>,
+                 <&clkc 93>;
+        clock-names = "core", "pclk";
+        #address-cells = <1>;
+        #size-cells = <0>;
+    };