[POWERPC] 4xx: Add 460EX PCIe support to 4xx pci driver

All this code is needed to properly initialize the 460EX PCIe host
bridge(s). We re-initialize all ports again, even though this has been done
in the bootloader (U-Boot) before. This way we make sure, that we always
run the latest init code in Linux and don't depend on code versions from
U-Boot.

Unfortunately all IBM/AMCC chips currently supported in this PCIe driver need
a different reset-/init-sequence.

Tested on AMCC Canyonlands eval board.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>