lib: sbi: Fix how to check whether the domain contains fw_region

Because firmware is split into rw/rx segments, it cannot be recorded
by a root_fw_region. This problem is solved by adding a flag
fw_region_inited to sbi_domain.

Signed-off-by: Xiang W <wxjstz@126.com>
Reviewed-by: Himanshu Chauhan <hchauhan@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>