ARM: dts: imx: Fix schema warnings for pwm-leds

The node names for devices using the pwm-leds driver follow a certain
naming scheme (now).  Parent node name is not enforced, but recommended
by DT project.

  DTC     arch/arm/boot/dts/imx53-ppd.dt.yaml
  CHECK   arch/arm/boot/dts/imx53-ppd.dt.yaml
arch/arm/boot/dts/imx53-ppd.dt.yaml: leds-brightness: 'alarm-brightness' does not match any of the regexes: '^led(-[0-9a-f]+)?$', 'pinctrl-[0-9]+'
        From schema: Documentation/devicetree/bindings/leds/leds-pwm.yaml
  DTC     arch/arm/boot/dts/imx6dl-cubox-i.dt.yaml
  CHECK   arch/arm/boot/dts/imx6dl-cubox-i.dt.yaml
arch/arm/boot/dts/imx6dl-cubox-i.dt.yaml: pwmleds: 'front' does not match any of the regexes: '^led(-[0-9a-f]+)?$', 'pinctrl-[0-9]+'
        From schema: Documentation/devicetree/bindings/leds/leds-pwm.yaml
  DTC     arch/arm/boot/dts/imx6dl-cubox-i-emmc-som-v15.dt.yaml
  CHECK   arch/arm/boot/dts/imx6dl-cubox-i-emmc-som-v15.dt.yaml
arch/arm/boot/dts/imx6dl-cubox-i-emmc-som-v15.dt.yaml: pwmleds: 'front' does not match any of the regexes: '^led(-[0-9a-f]+)?$', 'pinctrl-[0-9]+'
        From schema: Documentation/devicetree/bindings/leds/leds-pwm.yaml
  DTC     arch/arm/boot/dts/imx6dl-cubox-i-som-v15.dt.yaml
  CHECK   arch/arm/boot/dts/imx6dl-cubox-i-som-v15.dt.yaml
arch/arm/boot/dts/imx6dl-cubox-i-som-v15.dt.yaml: pwmleds: 'front' does not match any of the regexes: '^led(-[0-9a-f]+)?$', 'pinctrl-[0-9]+'
        From schema: Documentation/devicetree/bindings/leds/leds-pwm.yaml
  DTC     arch/arm/boot/dts/imx6sx-softing-vining-2000.dt.yaml
  CHECK   arch/arm/boot/dts/imx6sx-softing-vining-2000.dt.yaml
arch/arm/boot/dts/imx6sx-softing-vining-2000.dt.yaml: pwmleds: 'blue', 'green', 'red' do not match any of the regexes: '^led(-[0-9a-f]+)?$', 'pinctrl-[0-9]+'
        From schema: Documentation/devicetree/bindings/leds/leds-pwm.yaml

Signed-off-by: Alexander Dahl <post@lespocky.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>