arm64: dts: rockchip: add ARM Mali GPU node for RK3399 SoCs

Add Mali GPU device tree node for the RK3399 SoCs, with devfreq
opp table.

RK3399 and RK3399-OP1 SoCs have a different recommendation table with
gpu opp. Also, the ARM's mali driver found on
https://developer.arm.com/products/software/mali-drivers/midgard-kernel

.

Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>