drm/xe/uapi: Document DRM_XE_DEVICE_QUERY_HWCONFIG

Add a documentation on the content and format of when using query type
DRM_XE_DEVICE_QUERY_HWCONFIG. The list of keys can be found in IGT
under lib/intel_hwconfig_types.h.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: José Roberto de Souza <jose.souza@intel.com>
Acked-by: Mateusz Naklicki <mateusz.naklicki@intel.com>
Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>