lib: sbi: Domains can be registered only before finalizing domains

The domains are boot-time system-level partitions so we should
allow platform support to register domains only before hart
domain assignments are finalized.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>