Use named constant for default RAM behaviour

The ibex_top_tracing module takes a ram_cfg_i for something to pass
through to the RAM. Use the named zero (RAM_1P_CFG_DEFAULT) instead of
building it by hand: now we get the right width.

examples/simple_system/rtl/ibex_simple_system.sv

@@ -212,7 +212,7 @@ module ibex_simple_system (
 
       .test_en_i              ('b0),
       .scan_rst_ni            (1'b1),
-      .ram_cfg_i              ('b0),
+      .ram_cfg_i              (prim_ram_1p_pkg::RAM_1P_CFG_DEFAULT),
 
       .hart_id_i              (32'b0),
       // First instruction executed is at 0x0 + 0x80