powerpc/64: Honor swiotlb limit in coherent allocations

FSL PCI cannot directly address the whole lower 4 GiB due to
conflicts with PCICSRBAR and outbound windows, and thus
max_direct_dma_addr is less than 4GiB.  Honor that limit in
dma_direct_alloc_coherent().

Note that setting the DMA mask to 31 bits is not an option, since many
PCI drivers would fail if we reject 32-bit DMA in dma_supported(), and
we have no control over the setting of coherent_dma_mask if
dma_supported() returns true.

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