Maintainers.txt: Update maintainers for Ampere platforms

This adds Rebecca Cran as reviewer, updates myself as maintainer, and
Leif as reviewer. Also, removes Vu Nguyen and Thang Nguyen due to no
longer working on EDK2.

Signed-off-by: Nhi Pham <nhi@os.amperecomputing.com>
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>
Reviewed-by: Rebecca Cran <rebecca@os.amperecomputing.com>