lib: Check region base for merging in sbi_domain_root_add_memregion()

We can merge region B onto region A only if base of region A is
aligned to region A order + 1.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Xiang W <wxjstz@126.com>