leds: turris-omnia: Notify sysfs on MCU global LEDs brightness change

leds: turris-omnia: Notify sysfs on MCU global LEDs brightness change

Recall that on Turris Omnia, the LED controller has a global brightness
property, which allows the user to make the front LED panel dimmer.

There is also a button on the front panel, which by default is
configured so that pressing it changes the global brightness to a lower
value (unless it is at 0%, in which case pressing the button changes the
global brightness to 100%).

Newer versions of the MCU firmware support informing the SOC that the
brightness was changed by button press event via an interrupt.

Now that we have the turris-omnia-mcu driver, which adds support for MCU
interrupts, add the ability to inform the userspace (via a sysfs
notification) that the global brightness was changed.

Signed-off-by: Marek Behún <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Lee Jones <[email protected]>