ARM: dts: imx6sl: Add vivante gpu nodes

The imx6sl soc has gpu_2d and gpu_vg, no 3d support:

etnaviv-gpu 2200000.gpu: model: GC320, revision: 5007
etnaviv-gpu 2204000.gpu: model: GC355, revision: 1215

The IP blocks seem to be already supported.

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