riscv: Make SBI v0.2 the default SBI version

To work with latest OpenSBI release (v0.7 or above) that has the HSM
extension support, select the SBI v0.2 support by default.

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