UefiCpuPkg/PiSmmCpuDxeSmm: Refine casting result to bigger size

UefiCpuPkg/PiSmmCpuDxeSmm: Refine casting result to bigger size

The commit is a follow-up of commit 8491e30.

In file MpService.c line 786:
Pte[Index] = (UINT64)((UINTN)PageTable + EFI_PAGE_SIZE * (Index + 1)) |
mAddressEncMask ...

(Where PageTable is of type VOID*, Index is of type UINTN, mAddressEncMask
is of type UINT64 and Pte[Index] is of type UINT64.)

Since in this case, the code logic ensures that the expression will not
exceed the range of UINTN, the commit will remove the explicit type cast
'(UINT64)'.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>