ARM: dts: rockchip: fix mali gpu node on rk3288

The binding specifies the actual implementations only (mali-t760
for example) but not the arm,mali-midgard used in some vendor kernels.
So drop that compatible property from the rk3288 where it had slipped in.

Also fix the node name which should be a generic gpu@...

Signed-off-by: Heiko Stuebner <heiko@sntech.de>