MdePkg: Reference new definitions for Management Mode.

In PI 1.5 version, system management mode name(SMM) has been changed
to Management Mode(MM). It impacts the current code which still use
SMM/Smram/SMI keywords. This patch update the original files which
related to old SMM modules, also keep the compatible with old SMM
related drivers.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>