lib: sbi: Add regions merging when sanitizing domain region

As the domain will reject a new memory region which has a sub-regions
already in the domain, even the new region is bigger and has the same
flags. This problem can be solved by relaxing region restriction and
rechecking when adding and sanitizing domains.

Signed-off-by: Inochi Amaoto <inochiama@outlook.com>
Reviewed-by: Anup Patel <anup@brainfault.org>