powerpc/85xx: Rework SBC8548 pci_init_board to use common FSL PCIe code

Remove duplicated code in SBC8548 board and utilize the common
fsl_pcie_init_board().  We also now dynamically setup the LAWs for PCI
controllers based on which PCIe controllers are enabled.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Tested-by: Paul Gortmaker <paul.gortmaker@windriver.com>