Maintainers.txt: Add Platform and Silicon Intel maintainers

Enable maintainers for Platform/Intel/ subtree.
Enable maintainers for Silicon/Intel/ subtree.
This is consistent with Features/Intel/ subtree.

Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Sai Chaganty <rangasai.v.chaganty@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Signed-off-by: Isaac Oram <isaac.w.oram@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Sai Chaganty <rangasai.v.chaganty@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>