[dv] tb_cs_registers golden-model error in ePMP extension

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## Observed Behavior

DV test `lowrisc:ibex:tb_cs_registers` fails given `+ntb_random_seed=111`
```
Error, transaction:
Register transaction:
Operation: CSR Read
Address: PMPCfg0
Read data: 6981400
Expected rdata: 6981484
- ../src/lowrisc_ibex_tb_cs_registers_0/tb/tb_cs_registers.sv:111: Verilog $finish
Received $finish() from Verilog, shutting down simulation.
[Reg driver] drove: 4970 register transactions

//-------------//
// TEST FAILED //
//-------------//
```

## Expected Behavior

CS_Registers test should pass, RTL's dealing with mseccfg and pmpcfg is right.
At that point, mseccfg.MML is set, write a **new entry with exec privilege** to pmpcfg results in no change.

The provided golden-model in `base_register.cc` is wrong when handling CSRWrite to `pmpcfg`.
~~It should first check mseccfg.MML then write, rather than dealing with `// No reserved L/R/W/X values when MML Set`.~~

It should check whether the new entry is a "new rule with executable privilege".
This may be due to SMEPMP extension's evolving.

```c++
uint32_t PmpCfgRegister::RegisterWrite(uint32_t newval) {
uint32_t lock_mask = GetLockMask();
uint32_t read_value = register_value_;

register_value_ &= lock_mask;
register_value_ |= (newval & ~lock_mask);
register_value_ = HandleReservedVals(register_value_);

return read_value;
}
```

## Steps to reproduce the issue

```
fusesoc --cores-root=. run --target=sim --tool=verilator lowrisc:ibex:tb_cs_registers --run_options='+
ibex_tracer_enable=1 +ntb_random_seed=0'
```

## My Environment

**EDA tool and version:**

Verilator 4.210 2021-07-07 rev v4.210

**Operating system:**

Ubuntu Linux 22.04

**Version of the Ibex source code:**

8f4c75c5e43e61b994248307c814e01860303056

Start by reproducing the failure with the provided FuseSoC Verilator command and seed, then inspect PmpCfgRegister::RegisterWrite in base_register.cc alongside the failure at tb_cs_registers.sv:111. Compare the golden model's pmpcfg handling with the described mseccfg.MML and executable-privilege behavior. Done means the tb_cs_registers test passes for the reproducing seed.

Written by the indexing model from the issue text.