Allow for multiple synthesis directives in specials.

This is needed to specify timing constraints on some Lattice Diamond
library specials, like the EHXPLLL.

To keep backwards compatibility we allow the directive to still be a
single string. If it's not, we assume it's an iterable.

litex/gen/fhdl/specials.py

@@ -181,11 +181,18 @@ class Instance(Special):
                 r += "\t." + name_inst + "(" + name_design + ")"
         if not firstp:
             r += "\n"
-        if instance.synthesis_directive is not None:
-            synthesis_directive = "/* synthesis {} */".format(instance.synthesis_directive)
-            r += ")" + synthesis_directive + ";\n\n"
-        else:
-            r += ");\n\n"
+
+        directives = instance.synthesis_directive
+        if directives is None:
+            directives = []
+        elif type(directives) == str :
+            directives = [directives,]
+
+        r += ")";
+        for directive in directives:
+            r += "\n\t/* synthesis {} */".format(directive)
+        r += ";\n\n"
+
         return r