[otbn] Use libcst to extract implementations for docs

This isn't the full solution: we are going to want to do some
prettifying to make this more "pseudo-codeish". For example, we
currently have lines like

  val1 = state.gprs.get_reg(self.grs1).read_unsigned()
  state.gprs.get_reg(self.grd).write_unsigned(result)

which should probably be something more like

  val1 <- gprs[self.grs1]
  gprs[self.grd] <- val1

or similar. However, it does get rid of the duplicated definitions
from the YAML files and, if ugly, the definitions in the spec are now
at least clear.

Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>

util/build_docs.py

diff --git a/util/build_docs.py b/util/build_docs.py
index 477361f..4aa6e81 100755
--- a/util/build_docs.py
+++ b/util/build_docs.py
@@ -330,9 +330,11 @@
     otbn_dir = SRCTREE_TOP / 'hw/ip/otbn'
     script = otbn_dir / 'util/yaml_to_doc.py'
     yaml_file = otbn_dir / 'data/insns.yml'
+    impl_file = otbn_dir / 'dv/otbnsim/sim/insn.py'
 
     out_dir = config['outdir-generated'].joinpath('otbn-isa')
-    subprocess.run([str(script), str(yaml_file), str(out_dir)], check=True)
+    subprocess.run([str(script), str(yaml_file), str(impl_file), str(out_dir)],
+                   check=True)
 
 
 def hugo_match_version(hugo_bin_path, version):