arm64: dts: rockchip: add led support for Firefly-RK3399

This commit adds led support for the Firefly-RK3399. The board has two
leds, this commit enables them.

Signed-off-by: Shohei Maruyama <cheat.sc.linux@outlook.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>