AXI4 with pulp-platform axi

During use the pulp-axi component with AXI4 peripheral, the  parameter ATOP should be set to 0. 

Assign the atop signal of AXI4 master, slave to 0 especially in axi_width_converter, otherwise it will not work properly. 

Anyway, axi_atop_filter could also be used to filter the ATOP signal.