mailbox: stm32-ipcc: Update wakeup management

The wakeup specific IRQ management is no more needed to wake up the
stm32 platform. A relationship has been established between the EXTI and
the RX IRQ, just need to declare the EXTI interrupt instead of the
IPCC RX IRQ.

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