ipbus_freq_ctr_div entity is named just `freq_ctr_div`

[alessandrothea]

Nothing particularly wrong with it except for the fact that all other slave's names are prefixed with ipbus_ (and the .vhd file as well)

[tswilliams]

Following a discussion about naming conventions at the end of last year: We should not add the ipbus_ prefix to the entity name - since none of the ports are ipbus records. Instead we should remove ipbus_ from the file's name, to match the entity name.