ARM: dts: stm32: remove the IPCC "wakeup" IRQ on stm32mp151

The stm32 ipcc mailbox driver supports only two interrupts (rx and tx), so
remove the unsupported "wakeup" one.
Note that the EXTI interrupt 61 has two roles : it is hierarchically linked
to the GIC IPCC "rx" interrupt, and it acts as a wakeup source.

Signed-off-by: Fabien Dessenne <fabien.dessenne@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>