dts: bindings: usb_audio: Add volume properties

Add volume feature specific fields. So remove this outdated description.

Signed-off-by: Saw Xu <Saw1993@126.com>

dts/bindings/usb/usb-audio-feature-volume.yaml

@@ -0,0 +1,37 @@
+# Copyright (c) 2023, LISTENAI
+# SPDX-License-Identifier: Apache-2.0
+#
+# Specific fields for USB volume control.
+
+description: USB volume control specific fields.
+
+compatible: "usb-audio-feature-volume"
+
+include: base.yaml
+
+properties:
+  volume-max:
+    type: int
+    default: 0x0A00
+    description: |
+      attention: this attribute is a signed value.
+      This attribute represents the maximum volume level.
+      The range from +127.9961 dB (0x7FFF) down to -127.9961 dB (0x8001).
+      Valid range: 0 - 0xFFFF
+  volume-min:
+    type: int
+    default: 0xBA00
+    description: |
+      attention: this attribute is a signed value.
+      This attribute represents the minimum volume level.
+      The range from +127.9961 dB (0x7FFF) down to -127.9961 dB (0x8001).
+      Valid range: 0 - 0xFFFF
+  volume-res:
+    type: int
+    default: 0x100
+    description: |
+      attention: this attribute can only take positive values.
+      This attribute represents the volume resolution(step).
+      1 = 1/256 dB or 0.00390625 dB.
+      0x100(256) = 1dB.
+      Valid range: 1 - 0x7FFF

dts/bindings/usb/usb-audio-hp.yaml

@@ -7,7 +7,7 @@ description: USB Audio headphones specific fields.
 
 compatible: "usb-audio-hp"
 
-include: usb-audio.yaml
+include: [usb-audio.yaml, usb-audio-feature-volume.yaml]
 
 properties:
   resolution:

dts/bindings/usb/usb-audio-hs.yaml

@@ -7,7 +7,7 @@ description: USB Audio headset specific fields.
 
 compatible: "usb-audio-hs"
 
-include: usb-audio.yaml
+include: [usb-audio.yaml, usb-audio-feature-volume.yaml]
 
 properties:
   mic-resolution:
@@ -167,7 +167,6 @@ properties:
     type: boolean
     description: |
       Enable Volume feature.
-      Currently not supported.
   hp-feature-tone-control:
     type: boolean
     description: |