[U-Boot] dual licensed files

[Roger Meier]

Thank you Wolfgang for simplifying license handling!

What about dual licensed files? What do you prefer?

SPDX-License-Identifier:        GPL-2.0+
SPDX-License-Identifier:        BSD-2-Clause
	or
SPDX-License-Identifier:        GPL-2.0+, BSD-2-Clause
	Other seperator?

All the best!
-roger