powerpc/pci: Change how re-assigning resouces work

When PCI_REASSIGN_ALL_RSRC is set, we used to clear all bus resources
at the beginning of survey and re-allocate them later.

This changes it so instead, during early fixup, we mark all resources
as IORESOURCE_UNSET and move them down to be 0-based.

Later, if bus resources are still unset at the beginning of the survey,
then we clear them.

This shouldn't impact the re-assignment case on 4xx, but will enable
us to have the platform do some custom resource assignment before the
survey, by clearing individual resources IORESOURCE_UNSET bit.

Also limits the clutter in the kernel log from fixup when re-assigning
since we don't care about the offset applied to the BAR values in this
case.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>