MdePkg: Add FdtLib gmock support

Add Google Mock Library for FdtLib

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Signed-off-by: Jeff Brasen <jbrasen@nvidia.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Gua Guo <gua.guo@intel.com>