powerpc/fsl-pci: Limit ZONE_DMA32 to 2GiB on 64-bit platforms

FSL PCI cannot directly address the whole lower 4 GiB due to
conflicts with PCICSRBAR and outbound windows.  By the time
max_direct_dma_addr is set to the precise limit, it will be too late to
alter the zone limits, but we should always have at least 2 GiB mapped
(unless RAM is smaller than that).

Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: Shaohui Xie <Shaohui.Xie@freescale.com>