Fix JSON for muxes using udp_mux_4to2.

Fixes to the model for `udp_mux_4to2` cause the JSON layout output to now
include the cell.

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>
tree: 4ae2743a1a8eeecb20be333baf9860b343468795
  1. .gitignore
  2. LICENSE
  3. README.rst
  4. cells/
  5. models/
  6. tech/
  7. timing/