boot/at91bootstrap3: add license information

This patch adds licensing informations to at91bootstrap3

[Thomas:
 - change from "BSD-2c with endorsement clause" to "Atmel License",
   because the license of AT91Bootstrap is neither a BSD-2c nor a
   BSD-3c: it does not carry the second clause of the BSD-2c or BSD-3c
   licenses giving the condition of redistribution of the binary form.]

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>