Move immediate decoder to separate module

serv.core

@@ -12,6 +12,7 @@ filesets:
       - rtl/serv_csr.v
       - rtl/serv_ctrl.v
       - rtl/serv_decode.v
+      - rtl/serv_immdec.v
       - rtl/serv_mem_if.v
       - rtl/serv_rf_if.v
       - rtl/serv_rf_ram_if.v