dt-bindings: arm: stm32: Add compatible for syscon tamp node

dt-bindings: arm: stm32: Add compatible for syscon tamp node

Since commit ad440432d1f9 ("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 documents the new compatible for stm32 SoC to support
TAMP registers access.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201014125441.2457-2-arnaud.pouliquen@st.com
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>