lib: sbi: Add sbi_domain_root_add_memregion() API

We should allow platform support to add more root memory regions
before domains are finalized. This will help platform support to
protect critical M-mode only resources.

This patch adds sbi_domain_root_add_memregion() API for above
described purpose.

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