arm64: dts: rockchip: enable the gpu on px30-evb

The px30 has a Mali Bifrost gpu, so enable it on the evb board
and connect it with its supplying regulator.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Link: https://lore.kernel.org/r/20191208145508.3124-3-heiko@sntech.de