riscv: Introduce SPL_SMP Kconfig option for U-Boot SPL

With SBI v0.2 HSM extension, only a single hart need to boot and
enter operating system. The booting hart can bring up secondary
harts one by one afterwards.

For U-Boot running in SPL, SMP can be turned on, while in U-Boot
proper, SMP can be optionally turned off if using SBI v0.2 HSM.

Introduce a new SPL_SMP Kconfig option to support this.

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