target/riscv: Enable privileged spec version 1.12

target/riscv: Enable privileged spec version 1.12

Virt machine uses privileged specification version 1.12 now.
All other machine continue to use the default one defined for that
machine unless changed to 1.12 by the user explicitly.

This commit enforces the privilege version for csrs introduced in
v1.12 or after.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Message-Id: <20220303185440.512391-7-atishp@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>