powerpc/pci: Add a platform hook after probe and before resource survey

Some platforms need to perform resource allocation using a custom algorithm
due to HW constraints, or may want to tweak things globally below a host
bridge. For example OPAL support for IODA will need to perform a
resource allocation pass that applies IODA specific segmentation
constraints to MMIO which cannot be done simply using the kernel generic
resource management code.

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