drm/xe/configfs: Add internal API to documentation

Add the internal configfs API like is done with other parts of the
driver. Also ensure the correct number of '=' chars are used for the
header.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20250528-engine-mask-v4-5-f4636d2a890a@intel.com


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