hw: Improve debug parameterization, fix debug CSR RW (#27)

* hw: Make `dcsr`, `dpc`, and `dscratch0` CSRs accessible

* hw: Fully gate debug support in Snitch core

* hw: Assert `irq_i.debug` is raised only if debug is supported

---------

Co-authored-by: dbekatli <[email protected]>
Co-authored-by: Luca Colagrande <[email protected]>