devlink: Add support for region snapshot delete command

Add support for DEVLINK_CMD_REGION_DEL used
for deleting a snapshot from a region. The snapshot ID is required.
Also added notification support for NEW and DEL of snapshots.

Signed-off-by: Alex Vesker <valex@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>