cache_ctrl: Generalise AXI offset generation

Created by: niwis

For XLEN = 64, some tools (e.g. VCS) still elaborate the offset generation block for XLEN = 32, throwing an elaboration error (illegal bit access). Hence, VCS can currently not build the 64-bit WB configuration of CVA6. Fix this by generating the AXI offset in an equivalent, parameter-agnostic and tool-friendly way.