ARM: dts: imx6: remove GPU subsystem nodes

They aren't needed by the etnaviv driver anymore and have been removed
from the binding.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>