ARM: dts: rockchip: enable ARM Mali GPU on rk3288-rock2-som

Add reference to the Mali GPU device tree node on the
rk3288-rock2-som platform.  Tested on a Radxa Rock2 Square board.

Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>