This enables the caller to initialize swiotlb with its own iotlb
memory late in the bootup.

See git commit eb605a57
"swiotlb: add swiotlb_tbl_map_single library function" which will
explain the full details of what it can be used for.

CC: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
[v1: Fold in smatch warning]
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>