[regtool] ral for single field register is different from multi field register

### Description

For single field register ral file contains:

```verilog
.add_hdl_path_slice(
"u_reg.u_.qs",
0, 18, 0, "BkdrRegPathRtl");
```

While for multi fields register it contains:

```verilog
.add_hdl_path_slice(
"u_reg.u__.qs",
0, 11, 0, "BkdrRegPathRtl");
```

The following is missing for single field register:

```
_
```

This cause automation issues.
Please add the missing part to single field registers.

Start by locating the regtool logic that generates RAL files, then compare the single-field and multi-field register paths shown in the issue. Check existing generation tests or generated fixtures if available. Done means single-field register paths include the field-name suffix consistently with multi-field registers and automation no longer sees different formats.

Written by the indexing model from the issue text.