UefiCpuPkg: Simplify the code to set smm page table as RO

UefiCpuPkg: Simplify the code to set smm page table as RO

Simplify the code to set memory used by smm page table as RO.
Since memory used by smm page table are in PageTablePool list,
we only need to set all PageTablePool as ReadOnly in smm page
table itself. Also, we only need to flush tlb once after
setting all page table pool as Read Only.

Signed-off-by: Dun Tan <dun.tan@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>