RISC-V: Simplify TranslationAlgorithm enum

What

Simplify the type encoding satp.mode from Result<TranslationAlgorithm, u64> to a simple enum TranslationAlgorithm

Why

This makes the encoding of satp.mode represantable with a simple enum which can be trivially converted to and from u8 in a C-style cast.

This will be relevant when transforming the structure of how all CSRs are represented.

Manually testing the MR

cd src/riscv && make test

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR