x86, swiotlb: add map_page and unmap_page

This is a preparation of struct dma_mapping_ops unification. We use
map_page and unmap_page instead of map_single and unmap_single.

This is sorta temporary workaround. We will move them to lib/swiotlb.c
to enable x86's swiotlb code to directly use them.

We will remove map_single and unmap_single hooks in the last patch in
this patchset.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>