target/riscv: Update the Hypervisor CSRs to v0.4

Update the Hypervisor CSR addresses to match the v0.4 spec.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>