Documentation/core-api: add swiotlb documentation

There's currently no documentation for the swiotlb. Add documentation
describing usage scenarios, the key APIs, and implementation details.
Group the new documentation with other DMA-related documentation.

Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Petr Tesarik <petr@tesarici.cz>
Signed-off-by: Christoph Hellwig <hch@lst.de>