powerpc/fsl-pci: get PCI init out of board files

As an alternative incremental starting point to Jia Hongtao's patchset,
get the FSL PCI init out of the board files, but do not yet convert to a
platform driver.

Rather than having each board supply a magic register offset for
determining the "primary" bus, we look for which PCI host bridge
contains an ISA node within its subtree.  If there is no ISA node,
normally that would mean there is no primary bus, but until certain
bugs are fixed we arbitrarily designate a primary in this case.

Conversion to a platform driver and related improvements can happen
after this, as the ordering issues are sorted out.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>