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

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) <robh@kernel.org>
Link: https://lore.kernel.org/r/20240517142245.178556-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>