forked from Mirrors/opensbi

With the introduction of the register_extensions callback the range members (extid_start and extid_end) may now change and it has become a bit subtle as to when a probe function should be implemented. Document all the members and their relationship to the register_extensions callback. Signed-off-by: Andrew Jones <ajones@ventanamicro.com> Reviewed-by: Anup Patel <anup@brainfault.org>