powerpc/pci: Use PCIe IP block revision register instead of compatible

Freescale PCIe IP block revision bigger than rev2.2 will also need
redefine the sequence of inbound windows. So change to use IP block
revision instead of compatible for the judgment.

Signed-off-by: Roy Zang <tie-fei.zang@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>