[tlgen] Set minimum device spacing on crossbar level

In PR #25108, we reduced the minimum address spacing between / base alignment of devices to 256 B, which is necessary for some crossbars. For many other crossbars, in particular those used by software above the lowest level and DMA engines, a minimum of 4 KiB (corresponding to the smallest page size) is more appropriate. This is also the recommendation of TileLink.

The goal of this issue is to make the minimum device spacing a parameter of the crossbar definition.

See https://github.com/lowRISC/opentitan/pull/25108#discussion_r1841931510 for background information.

Start with PR #25108 and the linked discussion for the background on address spacing and the relevant crossbar definitions. Make minimum device spacing configurable per crossbar, supporting the 256 B and 4 KiB cases described in the issue, and verify that the resulting definitions follow the TileLink recommendation.

Written by the indexing model from the issue text.