[rv_dm] rv_dm needs a UNR report

[rswarbrick]

Description

Looking at results from the nightlies, the FSM coverage is low. But it turns out that the missing coverage is not possible! The two main areas are:

• The tlul_lc_gate instance that gates SBA connections. This is not hitting the StFlush state. Getting to that state requires a flush request (through flush_req_i). But that port is tied to zero in rv_dm.
• In dmi_jtag, we're not seeing the DMIOpFailed state. Looking at the code, this is entered when there is a DTM_ERR response in the connection between the dap and dm_top. It turns out that the riscv debug module never generates DTM_ERR (the string isn't even in the code!), so this FSM state is presumably used if you happen to be connecting dmi_jtag to some other debug module. But we're not!

In both cases, I'd expect UNR to mark the states as unreachable.

There are two edges which I believe are possible (state_q going from Read or Write to Idle). I think this happens if we get a trst_n reset on the JTAG bus when the FSM is in the Read or Write state. That remains to be addressed, but won't be affected by a UNR report.

Note that the FSM coverage is currently reported as 75%, which is below the threshold needed for V2. Assuming my diagnosis is correct, the UNR report will jump the number well over the threshold.

[rswarbrick]

The two edges mentioned at the end of the initial report are possible to hit (I'm pleased to say). PR #24152 hits them.

[rswarbrick]

This was done by @moidx in 51852781d790a158784169effa7b785ba689612f.