x86, io_apic: Introduce set_affinity function pointer

With interrupt remapping a special function is used to
change the affinity of an IO-APIC interrupt. Abstract this
with a function pointer.

Signed-off-by: Joerg Roedel <joro@8bytes.org>
Acked-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>