Feature improve freq ctr

[jhegeman]

Recent experience shows that the exact frequency of the IPBus clock differs from board to board. This is no surprise, really, because this clock tends to be derived from PCIe, AXI, etc. clocks which are not necessarily very precise. It has a visible effect, though, on the frequencies reported by ipbus_freq_ctr, which uses the IPBus clock as reference.

The current modification adds a separate reference clock input to ipbus_freq_ctr, so that a more precise clock can be used if so desired.

NOTE: This does imply an interface change. There now is a new input port clk_ref to be connected.

[jhegeman]

Oops....

[tswilliams]

Superseded by #219