dt-bindings: gpu: mali-utgard: add rockchip,rk3328-mali compatible

The rk3328 quad-core Cortex A53 uses a Mali-450MP2 with 2 PPs, so
add a compatible for it.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Rob Herring <robh@kernel.org>