ARM: dts: at91sam9xe: use SPDX-License-Identifier

ARM: dts: at91sam9xe: use SPDX-License-Identifier

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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

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>