drm/xe: Make xe_gt_freq part of the Documentation

drm/xe: Make xe_gt_freq part of the Documentation

The documentation was created with the creation of the component,
however it has never been actually shown in the actual Documentation.

While doing this, fixes the identation style, to avoid new warnings
while building htmldocs.

Fixes: bef52b5c7a19 ("drm/xe: Create a xe_gt_freq component for raw management and sysfs")
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20250521165146.39616-3-rodrigo.vivi@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>