[sival, kmac] No EDN timeout in fpga_cw340_sival_rom_ext

### Description

Hi,

`kmac_error_conditions_test` and `kmac_entropy` test both set the `entropy_wait_timer = 1` in the KMAC configuration such that the `WaitTimerExpired` error is provoked. This works for the `fpga_cw340_sival` target. However, when switching to `fpga_cw340_sival_rom_ext`, @nbdd0121 noticed that the `WaitTimer` never times out.

The failing test can be found [here](https://github.com/lowRISC/opentitan/blob/0fa5019bae516e4407fef47b5fedd5ee58304b8b/sw/device/tests/kmac_error_conditions_test.c#L177).

Start with sw/device/tests/kmac_error_conditions_test.c at the failing test around line 177, then compare the fpga_cw340_sival and fpga_cw340_sival_rom_ext target configurations and their WaitTimer behavior. Run the KMAC error-condition and entropy tests; done means entropy_wait_timer = 1 provokes WaitTimerExpired on the ROM-ext target as it does on the regular target.

Written by the indexing model from the issue text.