Maintainers.txt: Change SimicsOpenBoardPkg Maintainer

To help keep edk2-platforms healthy, I would like to
offer to maintain SimicsOpenBoardPkg, SimicsX58SktPkg,
and SimicsIch10Pkg. The current maintainer for those
packages has changed jobs and is no longer active in
the community.

Cc: Agyeman Prince <prince.agyeman@intel.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
Reviewed-by: Prince Agyeman <prince.agyeman@intel.com>
Acked-by: Leif Lindholm <leif@nuviainc.com>