Maintainers: assume co-maintainership of StandaloneMmPkg

Maintainers: assume co-maintainership of StandaloneMmPkg

Achin has indicated that he no longer has the bandwidth available to
co-maintain StandaloneMmPkg, and has asked Sami and me to step in.

Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Jiewen Yao <Jiewen.Yao@intel.com>
Reviewed-by: Achin Gupta <achin.gupta@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>