arm: dts: armada-xp-98dx: use SPDX-License-Identifier for prestara 98d SoCs

arm: dts: armada-xp-98dx: use SPDX-License-Identifier for prestara 98d SoCs

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.

[1] https://spdx.org/licenses/X11.html
[2] https://spdx.org/licenses/MIT.html

Cc: Chris Packham <chris.packham@alliedtelesis.co.nz>
Acked-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>