PMP improvement
Created by: masgia
- PMPCFG.A=NA4 not allowed
- access allowed in S/U mode when no PMP is configured (from the specs: if no PMP entry matches an S-mode or U-mode access, but at least one PMP entry is implemented, the access fails)
Note: the new signal between the commit_stage and the csr_registers is necessary to remove the combinatorial loop update_access_exception → csr_regfile_i/csr_exception_o → csr_exception_csr_commit → commit_stage_i/csr_exception_i → commit_stage_i/exception_o → commit_stage_i/csr_wdata_o[4] → csr_wdata_commit_csr[4] → csr_regfile_i/csr_wdata_i[4] → update_access_exception