[dv] Disable functional coverage in tl_agent/dv
[dv] Disable functional coverage in tl_agent/dv

We just need some basic check for tl_agent. Don't need to review
coverage.
With this update, coverage report will be shown as follows
|  SCORE  |  LINE  |  COND  |  TOGGLE  |  FSM  |  BRANCH  |  ASSERT  |
|:-------:|:------:|:------:|:--------:|:-----:|:--------:|:--------:|
|  -- %   |  -- %  |  -- %  |   -- %   | -- %  |   -- %   |   -- %   |

Signed-off-by: Weicai Yang <4fa8c7762ee4da95e3dee7dae2c3263ea16637f9@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan