Move simple address decoder to common/rtl/ dir.

hdl/common/rtl/Manifest.py

 files = ["dummy_ctrl_regs.vhd",
-         "dummy_stat_regs.vhd"]
+         "dummy_stat_regs.vhd",
+         "wb_addr_decoder.vhd"]

hdl/spec/rtl/Manifest.py

-files = ["spec_gn4124_test.vhd",
-         "wb_addr_decoder.vhd"]
+files = ["spec_gn4124_test.vhd"]
 
 modules = {"local" : ["../../common/rtl",
                       "../../gn4124core/rtl",