asix-sigma: add support for external clock

asix-sigma: add support for external clock

The 50MHz netlist supports the use of an external clock. Any of the 16
channels can use any of its edges to have another sample taken from all
the other pins. It's nice that the hardware does track timestamps, which
results in an exact reproduction of the input signals' timing with 20ns
resolution, although the clock is externally provided and need not have
a fixed rate.