xilinx PLL: allows to specify PLL output frequency (16 or 32 MHz)

servant.core

@@ -75,7 +75,7 @@ targets:
   nexys_a7:
     default_tool: vivado
     filesets : [mem_files, soc, nexys_a7]
-    parameters : [memfile, memsize]
+    parameters : [memfile, memsize, frequency=32]
     tools:
       vivado: {part : xc7a100tcsg324-1}
     toplevel : servix
@@ -110,6 +110,11 @@ parameters:
     description : Preload RAM with a hex file at runtime (overrides memfile)
     paramtype : plusarg
 
+  frequency:
+    datatype    : int
+    description : PLL output frequency
+    paramtype   : vlogparam
+
   memfile:
     datatype    : file
     description : Preload RAM with a hex file at compile-time