[TASK] Update documentation to include implemented CSRs that are missing from the documentation

[jstraus59]

Task Description

CV32E20 documentation makes no mention of the the following CSRs that are implemented:

• mconfigptr which is implemented in the RTL here
• mcounteren which is implemented in the RTL here
• menvcfg which is implemented in the RTL here
• menvcfgh which is implemented in the RTL here
• mstatush which is implemented in the RTL here

Note: this is an addon to task #153, which mentions only the missing documentation for the mcounteren CSR.

Description of Done

Update of the documentation.

Associated PRs

No response

[emgens]

These CSR's are only implemented in that they will return 0 if read; Currently the MHPCOUNTER[13-31] can also be read and return 0. Pull request 167 will fix that.

I believe that rather than update the documentation to match these partial implementations, we should remove these partial implementation.

[jstraus59]

@emgens wrote:

I believe that rather than update the documentation to match these partial implementations, we should remove these partial implementation.

Note that the RISC-V Instruction Set Manual, Volume II: Privileged Architecture, Document Version 20211203 referenced by the documentation states:

mconfigptr must be implemented, but it may be zero to indicate the configuration data structure
does not exist or that an alternative mechanism must be used to locate it.

and

In systems with U-mode, the mcounteren must be implemented, but all fields are WARL and may
be read-only zero, indicating reads to the corresponding counter will cause an illegal instruction
exception when executing in a less-privileged mode. In systems without U-mode, the mcounteren
register should not exist.

and

Register menvcfgh does not exist when MXLEN=64.
If U-mode is not supported, then registers menvcfg and menvcfgh do not exist.

and

Defined the mandatory RV32-only CSR mstatush, which contains most of the same fields as the upper 32 bits of RV64’s mstatus.

So the implementations of mconfigptr, mcounteren, menvcfgh and mstatush must not be removed, assuming U mode is present. But I feel they should be documented, otherwise it appears mandatory CSRs are missing!