mfd: cros_ec: Register keyboard backlight subdevice

Load cros_kbd_led_backlight when the EC reports EC_FEATURE_PWM_KEYB.
This makes cros_kbd_led_backlight work on machines without specific
ACPI or OF support for the keyboard backlight.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20240526-cros_ec-kbd-led-framework-v3-4-ee577415a521@weissschuh.net


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