tools: ffs-aio-example: add license information

Add missing information about license. Some people will probably want to
reuse this code in their projects released under variety of licenses. For this
reason this example is under Public Domain license to avoid GPL limitations.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>