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

Add reference to the Mali GPU device tree node on rk3288-veyron.
Tested on Minnie and Jerry boards.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>