ARM: dts: stm32: Fix schema warnings for pwm-leds on lxa-mc1

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/stm32mp157c-lxa-mc1.dt.yaml
  CHECK   arch/arm/boot/dts/stm32mp157c-lxa-mc1.dt.yaml
/home/alex/build/linux/arch/arm/boot/dts/stm32mp157c-lxa-mc1.dt.yaml: led-rgb: 'led-blue', 'led-green', 'led-red' do not match any of the regexes: '^led(-[0-9a-f]+)?$', 'pinctrl-[0-9]+'
        From schema: /home/alex/src/linux/leds/Documentation/devicetree/bindings/leds/leds-pwm.yaml

Signed-off-by: Alexander Dahl <post@lespocky.de>
Acked-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>