sysbus: Expose IRQ enumeration helpers

Sysbus devices can get their IRQ lines connected to other devices. It is
possible to figure out which IRQ line a connection is on and whether a sysbus
device even provides an IRQ connector at a specific offset.

This patch exposes helpers to make this information publicly accessible. We
will need it for the platform bus dynamic sysbus enumeration.

Signed-off-by: Alexander Graf <agraf@suse.de>
