drm/i915/bios: add legacy contents to common child device config

drm/i915/bios: add legacy contents to common child device config

Add legacy contents to common child device config, in preparation for
using a single child device config. Use unions where BDB versions of the
config differ. Use the naming from old_child_dev_config for legacy
fields.

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/b606456da4d52f1aedf383aab4275d81013d3178.1503600621.git.jani.nikula@intel.com