Unify VM code

Old vmem code had much 'cut-and-paste' replication for RV32 (Sv32) and (#408)

RV64 (Sv39, Sv48), and was scattered over several files.

New code unifies them into single set of parameterized functions
that works for RV32/RV64 and Sv32/Sv39/Sv48 (and is ready for Sv57).

Deleted old files:
    riscv_vmem_rv32.sail    riscv_vmem_rv64.sail
    riscv_vmem_sv32.sail    riscv_vmem_sv39.sail    riscv_vmem_sv48.sail
    riscv_pte.sail
    riscv_ptw.sail

Current files: all named riscv_vmem_*
    riscv_vmem.sail    (root file for vmem)
    riscv_vmem_common.sail
    riscv_vmem_pte.sail
    riscv_vmem_ptw.sail
    riscv_vmem_tlb.sail
    riscv_vmem_types.sail

Modified top-level Makefile accordingly.

Added documentation on new vmem code: doc/notes_Virtual_Memory.adoc