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

The DeviceTree binding document was converted to YAML in a patch that
is being merged through the devicetree tree, as such this patch needs to
be rewritten and is currently cause for a merge conflict.

This reverts commit 14ea1d04.
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

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

@@ -21,9 +21,6 @@ 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