Missing dependency in ipbus_freq_ctr.dep

[jhegeman]

The IPbus frequency counter component ipbus_freq_ctr is designed to work with input prescaled down by the freq_ctr_div component. The latter, however, is not included in the ipbus_freq_ctr.dep dependency file (https://github.com/ipbus/ipbus-firmware/blob/master/components/ipbus_slaves/firmware/cfg/ipbus_freq_ctr.dep).