ARM: dts: aspeed: Update license headers

In b2441318

 ("License cleanup: add SPDX GPL-2.0 license identifier
to files with no license") these files had the GPL-2.0 licence added
automatically. Update them to be GPL 2.0+ in line with other IBM kernel
contributions.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>