edk2-platforms: Maintainers.txt

Add maintainer and reviewer of ManageabilityPkg.

Signed-off-by: Abner Chang <abner.chang@amd.com>
Cc: Abdul Lateef Attar <abdattar@amd.com>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Isaac Oram <isaac.w.oram@intel.com>
Cc: Nickle Wang <nicklew@nvidia.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Nickle Wang <nicklew@nvidia.com>
Reviewed-by: Isaac Oram <isaac.w.oram@intel.com>
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>
Reviewed-by: Abdul Lateef Attar <abdattar@amd.com>