remoteproc: stm32: wakeup the system by wdg irq

If the device node defines the 'wakeup-source' property, use the WDG
exti IRQ as a wakeup source of the system.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Link: https://lore.kernel.org/r/1566833923-16718-3-git-send-email-fabien.dessenne@st.com


[bjorn: Replace ifdef with __maybe_unused for suspend/resume functions]
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>