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: 1ad64b031011bfdb3bf2fa9040ebaef84237a03a
  1. .gitignore
  2. LICENSE
  3. README.rst
  4. cells/
  5. models/
  6. tech/
  7. timing/