Code cleanups
Created by: e-matthews
Various code cleanups for CVA5:
- All LUTRAM and BRAM instantiations now use a common module
- Added core sub-directories to better organize rtl sources
- Various code cleanups: organization, variable names, etc.
- Improvements to the robustness of unit configuration and writeback organization
Tested with both Verilator environment and on Nexys board using cva5-accelerators setup