powerpc/vfio: Implement IOMMU driver for VFIO

powerpc/vfio: Implement IOMMU driver for VFIO

VFIO implements platform independent stuff such as
a PCI driver, BAR access (via read/write on a file descriptor
or direct mapping when possible) and IRQ signaling.

The platform dependent part includes IOMMU initialization
and handling.  This implements an IOMMU driver for VFIO
which does mapping/unmapping pages for the guest IO and
provides information about DMA window (required by a POWER
guest).

Cc: David Gibson <[email protected]>
Signed-off-by: Alexey Kardashevskiy <[email protected]>
Signed-off-by: Paul Mackerras <[email protected]>
Acked-by: Alex Williamson <[email protected]>
Signed-off-by: Benjamin Herrenschmidt <[email protected]>