ARM: dts: rockchip: add ARM Mali GPU node for rk3288

Add Mali GPU device tree node for the rk3288 SoC, with devfreq
opp table.

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