Add a new binding for SPI offload trigger sources.

Signed-off-by: David Lechner <dlechner@baylibre.com>
---

v4 changes: new patch in v4.

FWIW, this is essentially identical to the leds trigger-source binding.
---
 .../devicetree/bindings/spi/trigger-source.yaml    | 28 ++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/Documentation/devicetree/bindings/spi/trigger-source.yaml b/Documentation/devicetree/bindings/spi/trigger-source.yaml
new file mode 100644
index 000000000000..d64367726af2
--- /dev/null
+++ b/Documentation/devicetree/bindings/spi/trigger-source.yaml
@@ -0,0 +1,28 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/spi/trigger-source.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Trigger source providers for SPI offloads
+
+maintainers:
+  - David Lechner <dlechner@baylibre.com>
+
+description:
+  Each trigger source provider should be represented by a device tree node. It
+  may be e.g. a SPI peripheral chip or a clock source.
+
+properties:
+  '#trigger-source-cells':
+    description:
+      Number of cells in a source trigger. Typically 0 for nodes of simple
+      trigger sources. For nodes with more than one output signal, the first
+      cell be used to specify which output signal to use. If the same signal is
+      available on more than one pin, the second cell can be used to specify
+      which pin to use.
+    enum: [ 0, 1, 2 ]
+
+additionalProperties: true
+
+...

-- 
2.43.0