[rom_ctrl/dv] B2B same address read not tested yet

Rom_ctrl mem TL interface supports 2 outstanding requests.
https://github.com/lowRISC/opentitan/blob/316831f8c2199f2288dd57ac4aeaec3334b5dfb8/hw/ip/rom_ctrl/rtl/rom_ctrl.sv#L193

From the coverage result, looks like we haven't test B2B read on the same address.

I came across this because I tried to resolve this [TODO](https://cs.opensource.google/opentitan/opentitan/+/master:hw/dv/sv/tl_agent/tl_agent_cov.sv;l=73?q=tl_agent_cov&ss=opentitan%2Fopentitan:hw%2F) which uses a flag to disable above function coverage. Now I think we can probably remove this flag and the TODO as we should always cover 2 outstanding requests use the same address if it do support 2 or more outstanding requests.

cc: @GregAC @msfschaffner

> estimate 4

Start by inspecting the referenced interface logic in hw/ip/rom_ctrl/rtl/rom_ctrl.sv and the coverage TODO in hw/dv/sv/tl_agent/tl_agent_cov.sv. Use the coverage result to identify the missing back-to-back read case for two outstanding requests at the same address. Done means the case is covered and the disabling flag and TODO can be removed if no longer needed.

Written by the indexing model from the issue text.