PcAtChipsetPkg: PcAtChipsetPkg.dsc add UefiCpuLib LibraryClass

PcAtChipsetPkg: PcAtChipsetPkg.dsc add UefiCpuLib LibraryClass

In preparation for moving StandardSignatureIsAuthenticAMD to UefiCpuLib
in UefiCpuPkg, PcAtChipset/PcAtChipsetPkg.dsc needs LibraryClass
UefiCpuLib.
LocalApicLib|UefiCpuPkg/Library/BaseXApicLib/BaseXApicLib.inf will need
UefiCpuLib LibraryClass.  Likely most "real" platforms will be using
BaseX2XApicLib instance which already required UefiCpuLib.

Cc: Ray Ni <ray.ni@intel.com>
Signed-off-by: Garrett Kirkendall <garrett.kirkendall@amd.com>
Message-Id: <20200622131825.1352-2-Garrett.Kirkendall@amd.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>