arm64: dts: imx8mp: add GPU nodes

Add the DT nodes for both the 3D and 2D GPU cores found on the i.MX8MP.

etnaviv-gpu 38000000.gpu: model: GC7000, revision: 6204
etnaviv-gpu 38008000.gpu: model: GC520, revision: 5341
[drm] Initialized etnaviv 1.3.0 20151214 for etnaviv on minor 0

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Tested-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>