irqchip/stm32: Optimizes and cleans up stm32-exti irq_domain

- In stm32_exti_alloc function, discards irq_domain_set_info
  with handle_simple_irq. This overwrite the setting defined while init
  of generic chips. Exti controller manages edge irq type.
- Removes acking in chained irq handler as this is done by
  irq_chip itself inside handle_edge_irq
- removes unneeded irq_domain_ops.xlate callback

Acked-by: Ludovic Barre <ludovic.barre@st.com>
Tested-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Radoslaw Pietrzyk <radoslaw.pietrzyk@gmail.com>
Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>