pci/layerscape: Setup mmu-masters property for the PCIe

Setup mmu-masters property for the PCIe controllers. This would be
used by the Linux SMMU driver, while setting up stream ID table mappings
for the PCIe devices.

Signed-off-by: Varun Sethi <Varun.Sethi@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>