xen: introduce xen_dma_map/unmap_page and xen_dma_sync_single_for_cpu/device

Introduce xen_dma_map_page, xen_dma_unmap_page,
xen_dma_sync_single_for_cpu and xen_dma_sync_single_for_device.
They have empty implementations on x86 and ia64 but they call the
corresponding platform dma_ops function on arm and arm64.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

Changes in v9:
- xen_dma_map_page return void, avoid page_to_phys.
Git-commit: 7100b077ab4ff5fb0ba7760ce54465f623a0a763
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Ian Maund <imaund@codeaurora.org>