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

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

  imx50-kobo-aura.dtb: gpio-leds: 'on' does not match any of the regexes: '(^led-[0-9a-f]$|led)', 'pinctrl-[0-9]+'
  imx6dl-yapp4-draco.dtb: led-controller@30: 'chan@0', 'chan@1', 'chan@2' do not match any of the regexes: '^led@[0-8]$', '^multi-led@[0-8]$', 'pinctrl-[0-9]+'

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