pfb_fir_generic timing improvements

fanout support for enable signals at various points.
fanout control for registering wide busses.

Can build an 8-tap 2 pol 10 bit data in, 18 bit coefficient, 18 bit output, 2^16 fft bin,
asynchronous system at 325MHz on ROACH2 (2 ADCs, pfb_fir_generic feeding snapshot blocks).
When the same block is included into a full PFB (by adding an fft_wideband_real block with
suitable parameters), and targetted at 300MHz operation, 15 timing errors (of 312 total) are p
fb_fir_generic related.