platform/chrome: cros_kbd_led_backlight: allow binding through MFD

The ChromeOS EC used in Framework laptops supports the standard CrOS EC
keyboard backlight protocol.
However the firmware on these laptops doesn't implement the ACPI ID
GOOG0002 that is recognized by cros_kbd_led_backlight and they also
don't use device tree.

Prepare the existing cros_kbd_led_backlight driver to be probed through
the CrOS EC MFD device which works without ACPI or OF support.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20240526-cros_ec-kbd-led-framework-v3-3-ee577415a521@weissschuh.net


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