PCI: Add support for non-BAR ROMs

Platforms may provide their own mechanisms for obtaining ROMs. Add support
for using data provided by the platform in that case.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Seth Forshee <seth.forshee@canonical.com>