iommu/vt-d: Introduce intel_iommu=pasid28, and pasid_enabled() macro

As long as we use an identity mapping to work around the worst of the
hardware bugs which caused us to defeature it and change the definition
of the capability bit, we *can* use PASID support on the devices which
advertised it in bit 28 of the Extended Capability Register.

Allow people to do so with 'intel_iommu=pasid28' on the command line.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>