dt-bindings: soc: sti: st,sti-syscon: document codec node

st,stih407-core-syscfg syscon block comes with a child - the audio
codec, whose binding is still in TXT format. Document the child to fix
dtbs_check validation errors, while allowing later the binding to be
converted to DT schema:

stih410-b2260.dtb: core-syscfg@92b0000: 'sti-sasg-codec' does not match any of the regexes: 'pinctrl-[0-9]+'

Reviewed-by: Rob Herring (Arm) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Krzysztof Kozlowski <[e-mail]>

Documentation/devicetree/bindings/soc/sti/st,sti-syscon.yaml

···
   reg:
     maxItems: 1
 
+  sti-sasg-codec:
+    description: STi internal audio codec
+    type: object
+    additionalProperties: true
+
+    properties:
+      compatible:
+        const: st,stih407-sas-codec
+
 required:
   - compatible
   - reg