arm64: dts: rockchip: add VPU device node for RK3399

Add the Video Processing Unit node for the RK3399 SoC.

Also, fix the VPU IOMMU node, which was disabled and lacking
its power domain property.

Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>