drm/i915/icl: update VBT's child_device_config flags2 field

Some bits from the flags2 field are going to be used in the next
patches, so replace the whole-byte definition with the actual bits and
document their versions.

This patch is based on a patch by Animesh Manna.

Cc: Animesh Manna <animesh.manna@intel.com>
Credits-to: Animesh Manna <animesh.manna@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180614221018.19044-2-paulo.r.zanoni@intel.com