Changes

gateware: Rework the low-level line state monitoring ticks

The 'tick' system was meant to allow the firmware to monitor the
low level state of the line, beyong just "not aligned", but it was
never really useful in the present state.

Now we have more tick source available (but only one can be measured
at the same time). More specifically, the firmware can monitor the
presence/absence of raw pulses (LOS condition), it can also monitor
the number of '1' pulses to detect an AIS signals, or simply the rate
of RX pulses (to tune the local oscillator to incoming signal rate).

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Change-Id: I31fa8a717a26643f1b6ca5644bee1d8c41a2512e