Minor update to packm_cxk kernels.

Details:
- Changed m and n dimension parameter names to panel_dim and panel_len,
  respectively, in packm_cxk, packm_cxk_3m, packm_cxk_4m kernel wrapper
  functions. This makes the code a little easier to read since "m" and
  "n" have connotations that are not applicable here.
- Comment updates.