swiotlb-xen: introduce xen_swiotlb_set_dma_mask

Implement xen_swiotlb_set_dma_mask, use it for set_dma_mask on arm.

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

diff --cc arch/arm/xen/mm.c
Simple merge

diff --cc drivers/xen/swiotlb-xen.c
Simple merge

diff --cc include/xen/swiotlb-xen.h
Simple merge