RISC-V: CONFIG_SPL_OPENSBI_SCRATCH_OPTIONS description

Describe which numeric values can be used for as scratch options for
OpenSBI.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
Reviewed-by: Anup Patel <anup@brainfault.org>