There is currently only very experimental support for interpolation onto sub entities (facets, edges, vertices).
However, you can use: dolfinx.mesh.compute_incident_entities to only interpolate onto cells containing a set of facets.
See the Documentation of compute_indicident_entities for the documentation of the input, or dolfinx/test_refinement.py at 621d1479adf1e1a8beb9eed86cc8ad3de963f223 · FEniCS/dolfinx · GitHub for an example (which computes the edges from a set of cells).
You can then pass the cells into u.interpolate as the second argument, ref: dolfinx.fem — DOLFINx 0.4.2.0 documentation