UefiCpuPkg/PiSmmCpu: Enable 5L paging only when phy addr line > 48

UefiCpuPkg/PiSmmCpu: Enable 5L paging only when phy addr line > 48

Today's behavior is to enable 5l paging when CPU supports it
(CPUID[7,0].ECX.BIT[16] is set).

The patch changes the behavior to enable 5l paging when two
conditions are both met:
1. CPU supports it;
2. The max physical address bits is bigger than 48.

Because 4-level paging can support to address physical address up to
2^48 - 1, there is no need to enable 5-level paging with max
physical address bits <= 48.

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