Add support in `CSR` configuration for user-defined access types

[mkorbel1]

Motivation

Different users of CSRs may have their own special ways of managing access types for registers. Sometimes that might imply a different way of accessing in generated hardware, and other times it might just be a matter of configuration for similar hardware.

Desired solution

Add a way for users to extend the types of access types supported for CSR configuration and hardware generation.

Alternatives considered

No response

Additional details

No response

[mkorbel1]

Perhaps making CsrAccess a base class that supports overrideable hardware generation? Maybe it should incorporate all the isdoor<Read/Write/able stuff too