ARM: dts: rockchip: enable ARM Mali GPU on rk3288-evb

Add reference to the Mali GPU device tree node on rk3288-evb.
Tested on rk3288-evb-act8846 board.

Signed-off-by: Jacob Chen <jacob-chen@iotwrt.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>