[Issue #38] RISC-V csrd rework

[HanyzPAPU]

I've reworked the csrd command to dump only selected CSRs.

I've also implemented a new subcommand feature for csrd - you can specify a subcommand instead of the CSR name/number which prints out a subset of CSRs.

The implemented subcommands are:

• mmode: prints all M-mode CSRs
• smode: prints all S-mode CSRs
• counters: prints all counter and counter setup CSRs
• all: prints all CSRs

The csrd command without any parameter now dumps the following CSRs:

• mstatus
• medeleg
• mideleg
• mie
• mtvec
• mcounteren
• mscratch
• mepc
• mcause
• mtval
• mip
• sstatus
• sie
• stvec
• scounteren
• sscratch
• sepc
• scause
• stval
• sip
• satp
• scyclecmp

A discussion about the included CSRs is welcome, I've chosen them arbitrarily by removing the counters, configuration and debug CSRs.

Note that this PR follows PR https://github.com/d-iii-s/msim/pull/60.

[vhotspur]

Merging, I think the selection is OK for now (we can fine tune it based on feedback from next semester labs).