arm64: dts: rockchip: add backlight support for rk3399 evb board

Add backlight node for evb board, perpare for panel device node.

Signed-off-by: Yakir Yang <ykk@rock-chips.com>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>