ASoC: dt-bindings: cirrus,cs35l41: Document the cirrus,subsystem-id property

Add new property: cirrus,subsystem-id
This new property is used to uniquely identify the system if device
tree is used, to allow the driver to select the correct firmware and
tuning for the system.

The DSP driver searches for a compatible firmware (and tuning) based on
what it is able to read from the hardware.

However, the SSID is based on the system, and cannot be read from the
hardware, therefore it needs to be read from the Device Tree.

On ACPI-based systems, it is able to read this from the ACPI _SUB
property, and to maintain compatibility with the driver between ACPI
and Device Tree systems we need an equivalent property.

Signed-off-by: Stefan Binding <sbinding@opensource.cirrus.com>
Link: https://patch.msgid.link/20250917153722.94978-2-sbinding@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: Stefan Binding

Documentation/devicetree/bindings/sound/cirrus,cs35l41.yaml

@@ -151,6 +151,12 @@ properties:
     minimum: 0
     maximum: 5
 
+  cirrus,subsystem-id:
+    $ref: /schemas/types.yaml#/definitions/string
+    description:
+      Subsystem ID. If this property is present, it sets the system name,
+      used to identify the firmware and tuning to load.
+
 required:
   - compatible
   - reg