We already log a "Signaling PME" whenever the PME service driver claims a
Root Port.  In fact, we also log the same message for every device in the
hierarchy below the Root Port.

Log the "Signaling PME" once (only for the Root Port, since we can
trivially find out which devices are below the Root Port), and include the
IRQ number in the message to help connect the dots with /proc/interrupts.
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>