xen/arm: IRQ: Defer routing IRQ to Xen until setup_irq() call

xen/arm: IRQ: Defer routing IRQ to Xen until setup_irq() call

When an IRQ is handling by Xen, setup is done in 2 steps:
    - Route the IRQ to the current CPU and set priorities
    - Set up the handler

For PPIs, these steps are called on every cpu. For SPIs, they are only called
on the boot CPU.

Dividing the setup in two step complicates the code when a new driver is
added to Xen (for instance a SMMU driver). Xen can safely route the IRQ
when the driver sets up the interrupt handler.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>