ARM: dts: mmp3: Add the GPU

There's a GC2000 3D core accompanied by a GC300 2D core.

Link: https://lore.kernel.org/r/20200718205019.184927-13-lkundrak@v3.sk


Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>