powerpc/fsl: Setup PCI inbound window based on actual amount of memory

Previouslly we just always set the inbound window to 2G.  This was
broken for systems with >2G.  If a system has >=4G we will need
SWIOTLB support to handle that case.

We now allocate PCICSRBAR/PEXCSRBAR right below the lowest PCI outbound
address for MMIO or the 4G boundary (if the lowest PCI address is above
4G).

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