Implement CSR

[rjlv2]

Need to implement and test CSR instructions.

[rjlv2]

Currently simple registers act as placeholders for the actual implementation.

[harrysarson]

Worth noting: in the latest spec version (https://github.com/riscv/riscv-isa-manual/releases/latest) CSR registers are no longer in the base RV32I so we could possibly justify not supporting them.

I quote from chapter 9:

The counters and timers are no longer considered mandatory parts of the standard base
ISAs, and so the CSR instructions required to access them have been moved out of the base ISA
chapter into this separate chapte