lib: sbi: Optimize probe of srst/susp

No need to do a fully comprehensive count, just find a supported reset
or suspend type

Signed-off-by: Xiang W <wxjstz@126.com>
Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>