pci_auto: Downgrade prefetch if necessary

Legacy PCI devices, like qemu's Bochs VGA device, are allowed to have
prefetchable 32-bit BARs, while PCIe devices are not allowed to have
32-bit prefetchable BARs. Typically prefetchable BARs are 64-bit and
typically the prefetch MMIO window is also 64-bit and placed above
4GiB, as it's the case on qemu sbsa-ref.

Currently the U-Boot code assumes that prefetchable BARs are
64-bit BARs and always tries to assign them into the prefetch
MMIO window.

When a 32-bit BAR is marked as prefetch, but the prefetch area is
not within the first 4GiB of the address space, then downgrade the
BAR and place it in the non-prefetch MMIO window.

For prefetch BARs there's no downside on being placed in non prefetch
MMIO areas, besides the possible slower performance when a driver tries
to map it Write-Combine.

TEST: Fixes pci_auto on QEMU sbsa-ref fails to autoconfigure BAR0.

Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>

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