MdePkg: Add new MM MP Protocol definition.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1937

EFI MM MP Protocol is defined in the PI 1.5 specification.

The MM MP protocol provides a set of functions to allow execution of
procedures on processors that have entered MM. This protocol has the
following properties:
1. The caller can invoke execution of a procedure on a processor, other
than the caller, that has also entered MM. Supports blocking and
non-blocking modes of operation.
2. The caller can invoke a procedure on multiple processors. Supports
blocking and non-blocking modes of operation.

Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>

MdePkg/Include/Pi/PiMultiPhase.h

@@ -176,4 +176,20 @@ VOID
   IN OUT VOID  *Buffer
   );
 
+/**
+  The function prototype for invoking a function on an Application Processor.
+
+  This definition is used by the UEFI MM MP Serices Protocol.
+
+  @param[in] ProcedureArgument    The pointer to private data buffer.
+
+  @retval EFI_SUCCESS             Excutive the procedure successfully
+
+**/
+typedef
+EFI_STATUS
+(EFIAPI *EFI_AP_PROCEDURE2)(
+  IN VOID  *ProcedureArgument
+);
+
 #endif