Maintainers.txt: AdvancedFeaturePkg and ClevoOpenBoardPkg maintainers

Adds maintainers and reviewers for the AdvancedFeaturePkg and
ClevoOpenBoardPkg.

Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Sai Chaganty <rangasai.v.chaganty@intel.com>
Cc: Ankit Sinha <ankit.sinha@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Isaac W Oram <isaac.w.oram@intel.com>
Cc: Liming Gao <liming.gao@intel.com>

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>