ppc4xx: Dynamic configuration of 4xx PCIe mode as root or endpoint mode

This patch adds support for dynamic configuration of PCIe ports for the
AMCC PPC4xx boards equipped with PCIe interfaces. These are the PPC440SPe
boards Yucca & Katmai and the 405EX board Kilauea.

This dynamic configuration is done via the "pcie_mode" environement
variable. This variable can be set to "EP" or "RP" for endpoint or
rootpoint mode. Multiple values can be joined via the ":" delimiter.
Here an example:

pcie_mode=RP:EP:EP

This way, PCIe port 0 will be configured as rootpoint, PCIe port 1 and 2
as endpoint.

Per default Yucca will be configured as:
pcie_mode=RP:EP:EP

Per default Katmai will be configured as:
pcie_mode=RP:RP:REP

Per default Kilauea will be configured as:
pcie_mode=RP:RP

Signed-off-by: Tirumala R Marri <tmarri@amcc.com>
Signed-off-by: Stefan Roese <sr@denx.de>