lib: sbi: Fix sbi_domain_root_add_memregion() for merging memregions

We should decrement root_memregs_count by one after merging two
memregions otherwise new memregion added after a merge will be
appended after last sentinel memregion of zero order.

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