adding g_BUILD_SIMULATED_MAC generic to choose between physical and simulated Phy

* Modifying enclustra_ax3_pm3_infra.dep to pick up modified
enclustra_ax3_pm3_infra.vhd from  fmc-mtlu-gw repo rather than ipbus repo

* Removing unused ( and confusing ) top_enclustra_ax3_pm3.vhd

* Editing top_enclustra_tlu_v1e.vhd to pass downn g_BUILD_SIMULATED_MAC