ARM: dts: imx6: Switch NXP board dts to SPDX identifier

Adopt the SPDX license identifier headers to ease license compliance
management.

Most of the i.MX NXP reference board dts files have already been
converted, so switch the remaining ones.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>