arm64: dts: rockchip: enable gpu on rk3399-puma

Set the supplying regulator and enable the gpu node on the rk3399-puma som.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20191023223954.3139-1-heiko@sntech.de