Only include necessary LFSR primitive

We previously had a dependency on all primitives in Ibex, even though we
only depend on the LFSR primitive. Now that there's a more fine-grained
dependency available, we can use that.

This has the great benefit of restricting all lint tools to only the
code we're interested in, and not linting all primitives in OpenTitan
together with Ibex. This also helps tools like yosys, which aren't able
to parse all of OpenTitan's code yet.

ibex_core.core

@@ -10,9 +10,7 @@ filesets:
     depend:
       - lowrisc:prim:assert
       - lowrisc:prim:clock_gating
-      # TODO: Only lfsr is needed. Replace with a more specific dependency
-      # once available.
-      - lowrisc:prim:all
+      - lowrisc:prim:lfsr
       - lowrisc:ibex:ibex_pkg
       - lowrisc:ibex:ibex_icache
     files: