irqchip/gic: Only populate set_affinity for the root controller

Setting the affinity of an IRQ, it only applicable for the root
interrupt controller and so only populate this operator for the root
controller.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>