iommu: Retire iommu_capable()

With all callers now converted to the device-specific version, retire
the old bus-based interface, and give drivers the chance to indicate
accurate per-instance capabilities.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/d8bd8777d06929ad8f49df7fc80e1b9af32a41b5.1660574547.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>