powerpc: fsl_pci: Add forced PCI Agent enumeration

powerpc: fsl_pci: Add forced PCI Agent enumeration

The following commit prevents the MPC8548E on the XPedite5200 PrPMC
module from enumerating its PCI/PCI-X bus:

    powerpc/fsl-pci: use 'Header Type' to identify PCIE mode

The previous patch prevents any Freescale PCI-X bridge from enumerating
the bus, if it is hardware strapped into Agent mode.

In PCI-X, the Host is responsible for driving the PCI-X initialization
pattern to devices on the bus, so that they know whether to operate in
conventional PCI or PCI-X mode as well as what the bus timing will be.
For a PCI-X PrPMC, the pattern is driven by the mezzanine carrier it is
installed onto. Therefore, PrPMCs are PCI-X Agents, but one per system
may still enumerate the bus.

This patch causes the device node of any PCI/PCI-X bridge strapped into
Agent mode to be checked for the fsl,pci-agent-force-enum property. If
the property is present in the node, the bridge will be allowed to
enumerate the bus.

Cc: Minghuan Lian <Minghuan.Lian@freescale.com>
Signed-off-by: Aaron Sierra <asierra@xes-inc.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>