iommu: introduce the concept of DFN...

iommu: introduce the concept of DFN...

...meaning 'device DMA frame number' i.e. a frame number mapped in the IOMMU
(rather than the MMU) and hence used for DMA address translation.

This patch is a largely cosmetic change that substitutes the terms 'gfn'
and 'gaddr' for 'dfn' and 'daddr' in all the places where the frame number
or address relate to a device rather than the CPU.

The parts that are not purely cosmetic are:

 - the introduction of a type-safe declaration of dfn_t and definition of
   INVALID_DFN to make the substitution of gfn_x(INVALID_GFN) mechanical.
 - the introduction of __dfn_to_daddr and __daddr_to_dfn (and type-safe
   variants without the leading __) with some use of the former.

Subsequent patches will convert code to make use of type-safe DFNs.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>