dt-bindings: arm: stm32: Add compatibles for syscon nodes

Since commit ad440432 ("dt-bindings: mfd: Ensure 'syscon' has a more specific compatible")
it is required to provide at least 2 compatibles string for syscon node.
This patch document the missing compatibles for stm32 SoCs.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>