[hw,lcctrl,dv] Improve DV environment for muxed JTAG TAP/DMI

Nobody has claimed this yet.

When merging the TLUL DMI LC Ctrl functionality, the DV environment was added with several ifdefs, deciding between the existing JTAG TAP and the new TLUL DMI interface. This implementation is not the most beautiful one, but it works. It was decided to merge that first to block further RTL development and improve the DV environment separately.

@rswarbrick Proposed some improvements in https://github.com/lowRISC/opentitan/pull/24507#discussion_r1810618433

> I wonder whether it would make sense to give the agent something like a "configure_use_dmi" function? Then we wouldn't need all the ifdefs: we could just call it with the correct value after construction, avoiding tangling the config so much with the construction.
>
> Working through the items:
>
> For the abits field, I think we'd just need set_reset?
> For the n_bits default value override, this just goes through to the uvm_reg constructor. I think we can just use the larger value (with a comment)?
> For the address field, I think we could just use the larger value again. It's the top field in the register, so I don't think that would cause a problem.

In a [second note](https://github.com/lowRISC/opentitan/pull/24507#discussion_r1810618433) he suggested to factor the ifdef to:

> ```
> `ifdef USE_DMI_INTERFACE
> localparam bit use_dmi_interface = 1'b1;
> `else
> localparam bit use_dmi_interface = 1'b0;
> `endif
> ```

Start with the DV environment for the muxed JTAG TAP and TLUL DMI interfaces, then review the improvements discussed in PR 24507. Check how USE_DMI_INTERFACE controls construction and configuration, including abits, n_bits, and address fields. Done means reducing the conditional tangling while preserving both interface modes.

Written by the indexing model from the issue text.