arm64: dts: imx: align LED node names with dtschema

The node names should be generic and DT schema expects certain pattern:

  freescale/imx8mm-emcon-avari.dtb: leds: 'green', 'red' do not match any of the regexes: '(^led-[0-9a-f]$|led)', 'pinctrl-[0-9]+'

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