irqchip/stm32-exti: Map direct event to irq parent

EXTI lines are mainly used to wake-up system from CStop low power mode.
Currently, if a device wants to use a EXTI (direct) line as wakeup line,
it has to declare 2 interrupts:
 - one for EXTI used to wake-up system (with dedicated_wake_irq api).
 - one for GIC used to get the wake up reason inside the concerned IP.

This split is not really needed as each EXTI line is actually "linked " to
a GIC. So to avoid this useless double interrupt management in each
wake-up driver, this patch lets the STM32 EXTI driver abstract it by
mapping each EXTI line to his corresponding GIC.

Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200717140717.29606-1-alexandre.torgue@st.com