dt-bindings: remoteproc: stm32: add wakeup-source

Add the "wakeup-source" property: if the optional wdg interrupt is
defined, then this property may be defined too.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Link: https://lore.kernel.org/r/1566833923-16718-2-git-send-email-fabien.dessenne@st.com


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>