iommu-common: move to arch/sparc

This code is only used by sparc, and all new iommu drivers should use the
drivers/iommu/ framework. Also remove the unused exports.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: David S. Miller <davem@davemloft.net>
Reviewed-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>