drm/xe: Support for mmap-ing mmio regions

Allow the driver to expose hardware register spaces to userspace
through GEM objects with fake mmap offsets. This can be useful
for userspace-firmware communication, debugging, etc.

v2: Minor doc fix (CI)
v3: Enforce MAP_SHARED (Tejas)
    Add fault handler with dummy page (Tejas, Matt Auld)
    Store physical address instead of xe_mmio in the GEM object (MattB)
v4: Separate xe_mmio_gem from xe_mmio and make it private (MattB)

Signed-off-by: Ilia Levi <ilia.levi@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Link: https://lore.kernel.org/r/20250714122658.1803-1-ilia.levi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>