ARM: dts: imx6sx: Add GPU bindings

Add bindings for the Vivante GC400T GPU found in the i.MX6SX.

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