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.