SecurityPkg: Tcg2Smm: Switching from gSmst to gMmst

SecurityPkg: Tcg2Smm: Switching from gSmst to gMmst

This change replaced gSmst with gMmst to support broader compatibility
under MM environment for Tcg2Smm driver.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Qi Zhang <qi1.zhang@intel.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
Signed-off-by: Kun Qin <kun.q@outlook.com>
Reviewed-by: Jiewen Yao <Jiewen.yao@intel.com>
Message-Id: <MWHPR06MB310218F28C7AAF8DB375E963F3969@MWHPR06MB3102.namprd06.prod.outlook.com>