Enable the icache in coremark

When executed on a configuration without an icache the enable is safe
but has no effect.

examples/sw/benchmarks/coremark/ibex/core_portme.c

@@ -169,6 +169,7 @@ void portable_init(core_portable *p, int *argc, char *argv[]) {
     ee_printf("ERROR! Please define ee_u32 to a 32b unsigned type!\n");
   }
   p->portable_id = 1;
+  icache_enable(1);
 }
 /* Function : portable_fini
         Target specific final code