UefiCpuPkg: CpuIo2Smm: Move CpuIo2Smm driver to consume gMmst

UefiCpuPkg: CpuIo2Smm: Move CpuIo2Smm driver to consume gMmst

This change replaced gSmst with gMmst to support broader compatibility
under MM environment for CpuIo2Smm driver.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
Signed-off-by: Kun Qin <kun.q@outlook.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>