UefiCpuPkg/Cpuid.h: Update CPUID.7H.ECX structure for 5-level paging

Reserved6 is changed to Reserved7 because the bit width is changed.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>