[dv] CSR testbench fixes

[SamuelRiedel]

Fix tb_cs_registers golden model and testbench:

• Fix reset signal always being low under newer Verilator version
• Implement SMEPMP write suppression (rule 4b) in the golden model
• Fix incorrect PMP address register locking in the golden model

@ZhongYic00 and @brdr-life, does this resolve #2242 for you?

Closes #2242

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[SamuelRiedel]

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