lib: sbi: Add sbi_domain_memregion_init() API

This patch adds sbi_domain_memregion_init() helper API which can
be used by platform support to initialize a domain memory region
before adding it to the root domain.

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