LICENSES: add BSD-1-Clause license ...

Incoming sama5d3 bootstrap code has been ported from the at91bootstrap
project and is licensed under a 1-Clause BSD License. To remove
boilerplate legal text there, place the license at a fixed location
and only note SPDX-License-Identifier and Copyright in the source files.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>