UefiCpuPkg/CpuTimerLib: Update LIBRARY_CLASS of Base instance.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2832



Update LIBRARY_CLASS of BaseCpuTimerLib to remove the usage limitation,
otherwise the Base instance cannot be used in some types of modules.

Signed-off-by: Jason Lou <yun.lou@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>