riscv: Add Kconfig option for SBI v0.2

SBI v0.2 is more scalable and extendable to handle future needs
for RISC-V supervisor interfaces. For example, 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.

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