leds: ktd2692: add device tree bindings for ktd2692

This patch adds the device tree bindings for ktd2692 flash LEDs.
Add Optional properties of child node for Flash LED

Signed-off-by: Ingi Kim <ingi2.kim@samsung.com>
Acked-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Reviewed-by: Varka Bhadram <varkabhadram@gmail.com>
Acked-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>