mmu: Prevent elaboration of incompatible MMU version

Created by: niwis

Fix the VCS flow for 64-bit CVA6 by guarding incompatible MMU implementations. For instance, mmu_sv32 assumes PLEN > VLEN, which is not the case for RV64.

Example: https://github.com/openhwgroup/cva6/blob/720b59e2ca6e002b9b3a2c47ed4ad85b847978a3/src/mmu_sv32/cva6_mmu_sv32.sv#L212 causes for 64-bit CVA6:

(vlog-8607) Negative replication multiplier (-8)