powerpc/iommu: Add iommu_ops to report capabilities and allow blocking domains

Up until now PPC64 managed to avoid using iommu_ops. The VFIO driver
uses a SPAPR TCE sub-driver and all iommu_ops uses were kept in the
Type1 VFIO driver. Recent development added 2 uses of iommu_ops to the
generic VFIO which broke POWER:
  - a coherency capability check;
  - blocking IOMMU domain - iommu_group_dma_owner_claimed()/...

This adds a simple iommu_ops which reports support for cache coherency
and provides a basic support for blocking domains. No other domain types
are implemented so the default domain is NULL.

Since now iommu_ops controls the group ownership, this takes it out of
VFIO.

This adds an IOMMU device into a pci_controller (=PHB) and registers it
in the IOMMU subsystem, iommu_ops is registered at this point. This
setup is done in postcore_initcall_sync.

This replaces iommu_group_add_device() with iommu_probe_device() as the
former misses necessary steps in connecting PCI devices to IOMMU
devices. This adds a comment about why explicit iommu_probe_device() is
still needed.

The previous discussion is here:
  https://lore.kernel.org/r/20220707135552.3688927-1-aik@ozlabs.ru/
  https://lore.kernel.org/r/20220701061751.1955857-1-aik@ozlabs.ru/



Fixes: e8ae0e14 ("vfio: Require that devices support DMA cache coherence")
Fixes: 70693f47 ("vfio: Set DMA ownership for VFIO devices")
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Timothy Pearson <tpearson@raptorengineering.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
[mpe: Fix CONFIG_IOMMU_API=n build]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/2000135730.16998523.1678123860135.JavaMail.zimbra@raptorengineeringinc.com