lib: sbi: add priority for reset handler

Let's make system_reset_check returning priority instead of only
true/false. In that case 0 - means not supported, and anything above
means priority that makes existing reset handlers being used in first
place, unless it is decided to lower their priority.

The handler with the most priority wins.

Signed-off-by: Nikita Shubin <n.shubin@yadro.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>