MAINTAINERS: merge SWIOTLB SUBSYSTEM into DMA MAPPING HELPERS

Commit 78013eaa

 ("x86: remove the IOMMU table infrastructure")
refactored the generic swiotlb/swiotlb-xen setup into pci-dma.c, but
misses to adjust MAINTAINERS.

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains about
broken references.

As only include/linux/swiotlb.h is unique to the SWIOTLB SUBSYSTEM section
and Christoph is maintainer of DMA MAPPING HELPERS and SWIOTLB SUBSYSTEM,
just merge those two sections.

Leave the small architecture-dependent pieces to the arch maintainers.

Further, update the XEN SWIOTLB SUBSYSTEM to include all swiotlb-xen
headers and replace the pattern in drivers with the specific one file that
matches this pattern.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>