Maintainers.txt: Update for Ampere platforms

This change adds maintainers and reviewers for Ampere platforms.

Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>

Signed-off-by: Nhi Pham <nhi@os.amperecomputing.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>