arm64: dts: imx8mm-beacon-baseboard: Correct LED default state

There is no LED default state "none".  leds-gpio driver maps it to
"off", so correct them to fix dtbs_check warnings like:

  arch/arm64/boot/dts/freescale/imx8mm-beacon-kit.dt.yaml:
    leds: led0:default-state:0: 'none' is not one of ['on', 'off', 'keep']

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>