mpc8641hpcn: Change PCI MEM pci bus address

Now that the rest of u-boot can support it, change the PCI bus
address of the PCI MEM regions from 0x80000000 to 0xc0000000,
and use the same bus address for both PCI1 and PCI2.  This will
maximize the amount of PCI address space left over to map RAM
on systems with large amounts of memory.

Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>