arm64: dts: imx8qxp: add GPU nodes

Add the DT node for the GPU core found on the i.MX8QXP.

etnaviv-gpu 53100000.gpu: model: GC7000, revision: 6214
[drm] Initialized etnaviv 1.3.0 20151214 for etnaviv on minor 0

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>