arm: move the handle_irq_route function to the GICv3 module

arm: move the handle_irq_route function to the GICv3 module

The handle_irq_route function is not needed with the GICv2.

On the ARMv8 port we will not assign a virt_id to each CPU,
opting to use the MPIDR as much as we can from the start.
GICv3 will need heavier refactoring for this purpose; by moving
this function we can reuse the GICv2 code on ARMv8.

Signed-off-by: Antonios Motakis <antonios.motakis@huawei.com>
[Jan: implement stub in v2 to reduce #ifdefs]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>