UefiCpuPkg/SmmCpuFeaturesLib: Abstract PcdCpuMaxLogicalProcessorNumber

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=3218

Adds a new function called GetCpuMaxLogicalProcessorNumber() to
return the number of maximum CPU logical processors (currently
gUefiCpuPkgTokenSpaceGuid.PcdCpuMaxLogicalProcessorNumber).

This allows the the mechanism used to retrieve the CPU maximum
logical processor number to be abstracted from the logic that
needs the value.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20210217213227.1277-5-mikuback@linux.microsoft.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>