PCI: Add PCI ROM helper for platform-provided ROM images

It turns out that some UEFI systems provide apparently an apparently valid
PCI ROM BAR that turns out to contain garbage, so the attempt in 547b52463
to prefer the ROM from the BAR actually breaks a different set of machines.
As Linus pointed out, the graphics drivers are probably in the best
position to make this judgement, so this basically reverts 547b52463 and
f9a37be0f and adds a new helper function. Followup patches will add support
to nouveau and radeon for probing this ROM source if they can't find a ROM
from some other source.

[bhelgaas: added reporter and bugzilla pointers, s/f4eb5ff05/547b52463]
Reference: https://bugzilla.redhat.com/show_bug.cgi?id=927451
Reference: http://lkml.kernel.org/r/[email protected]
Reported-by: Mantas Mikulėnas <[email protected]>
Reported-by: Chris Murphy <[email protected]>
Signed-off-by: Matthew Garrett <[email protected]>
Signed-off-by: Bjorn Helgaas <[email protected]>

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