powerpc/8xxx: Update PCI outbound window addresses for 36-bit configs

In these configuration we expect to have large amounts of memory (> 4G)
and thus will bounce via swiotlb some region of PCI address space.

The outbound windows were wasting 512M of address space by leaving a
gap between the top of the outbound window and the 4G boundary.  By
moving the top of the outbound window up to the 4G boundary we can
reclaim the vast majority of the 512M (minus space needed for PEXCSRBAR)
and thus reduces the amount of memory we have to bounce.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>