ASoC: stm32: fix sync property description in SAI bindings

SAI sync property must be described in SAI subnodes
section, as it is a property of child node.
This patch fixes commit 14f0e5f8
"ASoC: stm32: Add synchronization to SAI bindings".

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>