vendor._gowin: Add 'add_preferences' override

Implement 'add_preferences' override for CST files, allowing customizing
physical placement of primitive.

amaranth/vendor/_gowin.py

@@ -175,6 +175,9 @@ class GowinPlatform(TemplatedPlatform):
     The environment is populated by running the script specified in the environment variable
     ``AMARANTH_ENV_APICULA``, if present.
 
+    Available overrides:
+        * ``add_preferences``: inserts commands at the end of the CST file.
+
     Build products:
         * ``{{name}}.fs``: binary bitstream.
 
@@ -186,6 +189,9 @@ class GowinPlatform(TemplatedPlatform):
     The environment is populated by running the script specified in the environment variable
     ``AMARANTH_ENV_GOWIN``, if present.
 
+    Available overrides:
+        * ``add_preferences``: inserts commands at the end of the CST file.
+
     Build products:
         * ``{{name}}.fs``: binary bitstream.
     """
@@ -346,6 +352,7 @@ def _osc_div(self):
                     IO_PORT "{{port_name}}" {{attr_name}}={{attr_value}};
                 {% endfor %}
             {% endfor %}
+            {{get_override("add_preferences")|default("# (add_preferences placeholder)")}}
         """,
     }