[dv/keymgr] Zero coverage of anchored netlist constants

Nobody has claimed this yet.

### Description

In IP-level `keymgr` coverage, for the instantiation of `u_seed_anchor` both line and score metrics are 0%.

The inputs of this anchor primitive are netlist constants, so maybe this is causing the issue:

https://github.com/lowRISC/opentitan/blob/04969982572fd460516479bd89ab8c5b3af56ead/hw/ip/keymgr/rtl/keymgr.sv#L100-L123

The coverage essentially says that continuous assignments at lines 15 and 16 are never reached:

https://github.com/lowRISC/opentitan/blob/04969982572fd460516479bd89ab8c5b3af56ead/hw/ip/prim_generic/rtl/prim_generic_buf.sv#L15-L16

cc: @cindychip

Start by inspecting the u_seed_anchor instantiation in hw/ip/keymgr/rtl/keymgr.sv at lines 100-123 and the continuous assignments in hw/ip/prim_generic/rtl/prim_generic_buf.sv at lines 15-16. Review the IP-level keymgr coverage report and determine why the anchored netlist constants produce zero line and score coverage. Done means the cause is addressed and the affected coverage no longer reports zero.

Written by the indexing model from the issue text.