ARM: dts: imx6: add Vivante GPU nodes

This adds the device nodes for 2D, 3D and VG GPU cores.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>