ARM: dts: imx6-karo: Replace license text comment with SPDX identifier

Replace verbatim license text with a `SPDX-License-Identifier`

The comment header mis-attributes this license to be "X11", but the
license text does not include the last line "Except as contained in this
notice, the name of the X Consortium shall not be used in advertising or
otherwise to promote the sale, use or other dealings in this Software
without prior written authorization from the X Consortium.". Therefore,
this license is actually equivalent to the SPDX "MIT" license (confirmed
by text diffing).

Cc: Lothar Waßmann <LW@KARO-electronics.de>
Acked-By: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Bence Csókás <csokas.bence@prolan.hu>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>