arm64: dts: marvell: armada-8080-db: use SPDX-License-Identifier
Follow the recent trend for the license description, and also fix the
wrongly stated X11 to MIT.
As already pointed on the DT ML, the X11 license text [1] is explicitly
for the X Consortium and has a couple of extra clauses. The MIT
license text [2] is actually what the current DT files claim.