arm64: dts: rockchip: add the gpu for px30

The px30 contains a Mali Bifrost gpu, so add the necessary core node
for it with interrupts and powerdomains.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Link: https://lore.kernel.org/r/20191208145508.3124-2-heiko@sntech.de