mailbox: stm32-ipcc: do not enable wakeup source by default

By default do not enable the wakeup source.
This lets the userspace application decide whether the wakeup source
shall be enabled or not.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>