iommu/dma: Clean up Kconfig

Although iommu-dma is a per-architecture chonce, that is currently
implemented in a rather haphazard way. Selecting from the arch Kconfig
was the original logical approach, but is complicated by having to
manage dependencies; conversely, selecting from drivers ends up hiding
the architecture dependency *too* well. Instead, let's just have it
enable itself automatically when IOMMU API support is enabled for the
relevant architectures. It can't get much clearer than that.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/2e33c8bc2b1bb478157b7964bfed976cb7466139.1660668998.git.robin.murphy@arm.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>