PCI: allocate bus resources from the top down

Allocate space from the highest-address PCI bus resource first, then work
downward.

Previously, we looked for space in PCI host bridge windows in the order
we discovered the windows.  For example, given the following windows
(discovered via an ACPI _CRS method):

    pci_root PNP0A03:00: host bridge window [mem 0x000a0000-0x000bffff]
    pci_root PNP0A03:00: host bridge window [mem 0x000c0000-0x000effff]
    pci_root PNP0A03:00: host bridge window [mem 0x000f0000-0x000fffff]
    pci_root PNP0A03:00: host bridge window [mem 0xbff00000-0xf7ffffff]
    pci_root PNP0A03:00: host bridge window [mem 0xff980000-0xff980fff]
    pci_root PNP0A03:00: host bridge window [mem 0xff97c000-0xff97ffff]
    pci_root PNP0A03:00: host bridge window [mem 0xfed20000-0xfed9ffff]

we attempted to allocate from [mem 0x000a0000-0x000bffff] first, then
[mem 0x000c0000-0x000effff], and so on.

With this patch, we allocate from [mem 0xff980000-0xff980fff] first, then
[mem 0xff97c000-0xff97ffff], [mem 0xfed20000-0xfed9ffff], etc.

Allocating top-down follows Windows practice, so we're less likely to
trip over BIOS defects in the _CRS description.

On the machine above (a Dell T3500), the [mem 0xbff00000-0xbfffffff] region
doesn't actually work and is likely a BIOS defect.  The symptom is that we
move the AHCI controller to 0xbff00000, which leads to "Boot has failed,
sleeping forever," a BUG in ahci_stop_engine(), or some other boot failure.

Reference: https://bugzilla.kernel.org/show_bug.cgi?id=16228#c43
Reference: https://bugzilla.redhat.com/show_bug.cgi?id=620313
Reference: https://bugzilla.redhat.com/show_bug.cgi?id=629933


Reported-by: Brian Bloniarz <phunge0@hotmail.com>
Reported-and-tested-by: Stefan Becker <chemobejk@gmail.com>
Reported-by: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>