leds: Add no-op gpio_led_register_device when LED subsystem is disabled

Some systems use 'gpio_led_register_device' to make an in-memory copy of
their LED device table so the original can be removed as .init.rodata.
When the LED subsystem is not enabled source in the led directory is not
built and so this function may be undefined. Fix this here.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>