dt-bindings: leds: intel,lgm: Add missing 'led-gpios' property

The example has 'led-gpio' properties, but that's not documented. As the
'gpio' form is deprecated, add 'led-gpios' to the schema and update the
example.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20221207204327.2810001-1-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>