[dv] Improve jtag_riscv_agent
[dv] Improve jtag_riscv_agent

Addressed comments in #9149
1. This high-level driver is like a vseq, don't to check reset everywhere
as the lower-level should finish the item immediately once reset occurs.
2. change `ref` to `output` as it doesn't need to be `ref`

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

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