RISC-V: Move the entire hart selection via lottery to SMP

The booting hart selection via lottery is only useful for SMP systems.
Moreover, the lottery selection is only necessary for systems using
spinwait booting method. It is better to keep the entire lottery
selection together so that it can be disabled in future.

Move the lottery selection code to under CONFIG_SMP.

Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>