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>