target/riscv: Define simpler privileged spec version numbering

target/riscv: Define simpler privileged spec version numbering

Currently, the privileged specification version are defined in
a complex manner for no benefit.

Simplify it by changing it to a simple enum based on.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Message-Id: <20220303185440.512391-2-atishp@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>