ARM: dts: meson8b: update with SPDX Licence identifier

While the text specifies "of the GPL or the X11 license" the actual
license text matches the MIT license as specified at [0]

[0] https://spdx.org/licenses/MIT.html



Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>