lib: sbi: Remove domains_root_regions() platform callback

We now have sbi_domain_root_add_memregion() which allows platform
support to add root domain regions at boot-time so let's remove
the domains_root_regions() platform callback which was added
for this purpose.

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