UefiCpuPkg: CpuIo2Smm: Abstract SMM specific functions into separate file

UefiCpuPkg: CpuIo2Smm: Abstract SMM specific functions into separate file

This change abstracts CpuIo2Smm driver entrypoint into separate file and
moves functions/definitions that are not substantially specific to
Traditional MM (SMM) into CpuIo2Mm.* in order to set ways for Standalone
MM support in the future.

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: Ray Ni <ray.ni@intel.com>