to_string() layout for CSR instructions is not correct

[jerralph]

to_string() layout for CSR instructions is not correct. For 0xf1402573, currently displaying like this:
`CSRRS X10_A0, X0_ZERO, -236,
Where -236 is the CSR.

First CRS shouldn't be negative number. Secondly they should be looked up based on the number to get a pretty name, like cycle, time, etc. Last but not least the it should be csrrs rd, csr, rs1.

In general CSRs need some attention/development in riscv-vip.