MdePkg/SmmGipDispatch2.h: SMM GpiNum meaning clarification

To support GPI number > 64, PI 1.4 clarified SMM GpiNum meaning as below:
typedef struct {
  UINT64 GpiNum;
} EFI_SMM_GPI_REGISTER_CONTEXT;
Instead of the bit mask of 64 possible GPIs, GpiNum is one of 2^64 possible
GPIs that can generate an SMI. A 0 corresponds to logical GPI[00],
1 corresponds to logical GPI[1], and so on, and GpiNum of N corresponds to
GPI[N], where N can span from 0 to 2^64-1.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17350 6f19259b-4bc3-4df7-8a09-765794883524