x86/ioapic: Ignore root bridges without a companion ACPI device

Some PCI root bridges don't have a corresponding ACPI device.
This can be the case on some old platforms. Don't call acpi_ioapic_add()
on these bridges because they can't support ioapic hotplug.

Reported-and-tested-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Rui Wang <rui.y.wang@intel.com>
Reviewed-by: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bhelgaas@google.com
Cc: helgaas@kernel.org
Cc: linux-acpi@vger.kernel.org
Cc: linux-pci@vger.kernel.org
Cc: rjw@rjwysocki.net
Cc: tony.luck@intel.com
Link: http://lkml.kernel.org/r/1473522046-31329-1-git-send-email-rui.y.wang@intel.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>