iommu: Move IOMMU_DOMAIN_BLOCKED global statics to ops->blocked_domain

iommu: Move IOMMU_DOMAIN_BLOCKED global statics to ops->blocked_domain

Following the pattern of identity domains, just assign the BLOCKED domain
global statics to a value in ops. Update the core code to use the global
static directly.

Update powerpc to use the new scheme and remove its empty domain_alloc
callback.

Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Sven Peter <sven@svenpeter.dev>
Link: https://lore.kernel.org/r/1-v2-bff223cf6409+282-dart_paging_jgg@nvidia.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>