dt-bindings: leds: Add LED_FUNCTION_FNLOCK

Newer laptops have FnLock LED.

Add a define for this very common function.

Signed-off-by: Gergo Koteles <soyer@irl.hu>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Lee Jones <lee@kernel.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/8ac95e85a53dc0b8cce1e27fc1cab6d19221543b.1712063200.git.soyer@irl.hu


Signed-off-by: Hans de Goede <hdegoede@redhat.com>