ARM: dts: rockchip: add eDP/panel display device nodes for rk3288-evb

The default eDP panel on RK3288 EVB board is LG LP079QX1-SP0V TFT LCD,
we haven't declared the panel regulator in the 'panel-simple' device
node here, so the specific board like ACT8846 / RK8080 need to support
the panel power supply.

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