powerpc/64: Limit ZONE_DMA32 to 4GiB in swiotlb_detect_4g()

A DMA zone is still needed with swiotlb, for coherent allocations.
This doesn't affect platforms that don't use swiotlb or that don't call
swiotlb_detect_4g().

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