dt-bindings: mailbox: remove the IPCC "wakeup" IRQ

The stm32 ipcc mailbox driver supports only two interrupts (rx and tx), so
remove the unsupported "wakeup" one.

Signed-off-by: Fabien Dessenne <fabien.dessenne@foss.st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>