drivers/iommu: Export core IOMMU API symbols to permit modular drivers

Building IOMMU drivers as modules requires that the core IOMMU API
symbols are exported as GPL symbols.

Signed-off-by: Will Deacon <will@kernel.org>
Tested-by: John Garry <john.garry@huawei.com> # smmu v3
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>