iommu/dma: add a new dma_map_ops of get_merge_boundary()

This patch adds a new dma_map_ops of get_merge_boundary() to
expose the DMA merge boundary if the domain type is IOMMU_DOMAIN_DMA.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>