UefiCpuPkg/PiSmmCpu: Change variable names and comments to follow SDM

UefiCpuPkg/PiSmmCpu: Change variable names and comments to follow SDM

Per SDM, for IA-32e 4-KByte paging, there are four layers in the page
table structure:
1. PML4
2. Page-Directory-Pointer Table (PDPT)
3. Page-Directory (PD)
4. Page Table (PT)

The patch changes the local variable names and comments to use
"PML4", "PDPT", "PD", "PT" to better align to terms used in SDM.

There is no functionality impact for this change.

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