Moved decision of legal CSR accesses from decoder to cs_registers.
Created by: silabs-oysteink
Also killing CSR writes with csr_en instead of changing csr_op. EX stage will deassert write enables and set ex_wb_pipe.illegal_insn in case of illegal CSR instructions.
SEC equivalent under the following assumption:
- No illegal CSR instructions (csr_en would stay high until EX, causing stalls)
Signed-off-by: Oystein Knauserud Oystein.Knauserud@silabs.com