OvmfPkg: Add MicrocodeLib in DSC files.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3303



Signed-off-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>