leds: Add ChromeOS EC driver

The ChromeOS Embedded Controller exposes an LED control command.
Expose its functionality through the leds subsystem.

The LEDs are exposed as multicolor devices.
A hardware trigger, which is active by default, is provided to let the
EC itself take over control over the LED.

The driver is designed to be probed via the cros_ec mfd device.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20240613-cros_ec-led-v3-4-500b50f41e0f@weissschuh.net


Signed-off-by: Lee Jones <lee@kernel.org>