NTB: add support for hotplug feature

AMD NTB support hotplug under B2B mode. NTB will trigger link
up/down interrupt event when doing plug add/remove, this patch
implements the two interrupt event to support B2B hotplug function.

Signed-off-by: Xiangliang Yu <Xiangliang.Yu@amd.com>
Signed-off-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Acked-by: Allen Hubbe <Allen.Hubbe@dell.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>