PCI: Add 'nodomains' boot option, and pci_domains_supported global

* Introduce pci_domains_supported global, hardcoded to zero if
  !CONFIG_PCI_DOMAINS.

* Introduce 'nodomains' boot option, which clears pci_domains_supported
  on platforms that enable it by default (x86, x86-64, and others when
  they are converted to use this).

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>