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>

Documentation/devicetree/bindings/remoteproc/stm32-rproc.txt

@@ -21,6 +21,9 @@ Required properties:
 
 Optional properties:
 - interrupts:	Should contain the watchdog interrupt
+- wakeup-source: Flag indicating whether remoteproc can wake up the system by
+		the watchdog interrupt. Only meaningful if the "interrupts"
+		property is defined.
 - mboxes:	This property is required only if the rpmsg/virtio functionality
 		is used. List of phandle and mailbox channel specifiers:
 		- a channel (a) used to communicate through virtqueues with the