[prim] Add a missing ROM_CFG_DEFAULT to prim_rom_pkg.sv

This is needed if you want to wire up a module containing a ROM but
with the default config. (It mirrors e.g. RAM_1P_CFG_DEFAULT)

Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
diff --git a/hw/ip/prim/rtl/prim_rom_pkg.sv b/hw/ip/prim/rtl/prim_rom_pkg.sv
index c81d578..1ac84ee 100644
--- a/hw/ip/prim/rtl/prim_rom_pkg.sv
+++ b/hw/ip/prim/rtl/prim_rom_pkg.sv
@@ -10,4 +10,6 @@
     logic [3:0] cfg;
   } rom_cfg_t;
 
+  parameter rom_cfg_t ROM_CFG_DEFAULT = '0;
+
 endpackage // prim_rom_pkg