lib: sbi: Ensure domidx_to_domain_table is null-terminated

sbi_domain_for_each() requires domidx_to_domain_table[] to be
null-terminated. Allocate one extra element which will always
be null.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>