riscv: Add SMP Kconfig option dependency for U-Boot proper

U-Boot proper running in S-mode only need SMP support when using
SBI v0.1. With SBI v0.2 HSM extension, it does not need implement
multicore boot in U-Boot proper.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>