drm/i915/bios: document BDB versions of child device config fields

drm/i915/bios: document BDB versions of child device config fields

Document everything that was introduced after version 155, which seems
to be the baseline for some of the later documentation. No functional
changes.

Cc: Animesh Manna <animesh.manna@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/11e8200e750eea13604b2d21e56b37cd5e6d9ab0.1503600621.git.jani.nikula@intel.com