drm/xe/migrate: Add kerneldoc for the migrate subsystem

Add kerneldoc for structs and external functions.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>