ASoC: dt-bindings: simple-card: document new system-clock-fixed flag

Document the new system-clock-fixed flag, which can be used to specify
that the driver cannot or should not allow the clock frequency of the
mapped clock to be modified.
Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Link: https://lore.kernel.org/r/20220120195832.1742271-6-robert.hancock@calian.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/simple-card.yaml

@@ -48,6 +48,15 @@ definitions:
       It is useful for some aCPUs with fixed clocks.
     $ref: /schemas/types.yaml#/definitions/flag
 
+  system-clock-fixed:
+    description: |
+      Specifies that the clock frequency should not be modified.
+      Implied when system-clock-frequency is specified, but can be used when
+      a clock is mapped to the device whose frequency cannot or should not be
+      changed. When mclk-fs is also specified, this restricts the device to a
+      single fixed sampling rate.
+    $ref: /schemas/types.yaml#/definitions/flag
+
   mclk-fs:
     description: |
       Multiplication factor between stream rate and codec mclk.
@@ -134,6 +143,8 @@ definitions:
         $ref: "#/definitions/system-clock-frequency"
       system-clock-direction-out:
         $ref: "#/definitions/system-clock-direction-out"
+      system-clock-fixed:
+        $ref: "#/definitions/system-clock-fixed"
     required:
       - sound-dai