ARM: dts: imx6: remove undefined linux,default-trigger source

ARM: dts: imx6: remove undefined linux,default-trigger source

Set gpio/off to none for linux,default-trigger.
Fix below CHECK_DTBS warnings:
arch/arm/boot/dts/nxp/imx/imx6dl-phytec-mira-rdk-nand.dtb: user-leds (gpio-leds): user-led1:linux,default-trigger: 'oneOf' conditional failed, one must be fixed:
        'gpio' is not one of ['backlight', 'default-on', 'heartbeat', 'disk-activity', 'disk-read', 'disk-write', 'timer', 'pattern', 'audio-micmute', 'audio-mute', 'bluetooth-power', 'flash', 'kbd-capslock', 'mtd', 'nand-disk', 'netdev', 'none', 'rc-feedback', 'torch', 'usb-gadget', 'usb-host', 'usbport']

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>