arm64: dts: imx8mm: Add GPU nodes for 2D and 3D core

According to the documents, the i.MX8M-Mini features a GC320 and a
GCNanoUltra GPU core. Etnaviv detects them as:

	etnaviv-gpu 38000000.gpu: model: GC600, revision: 4653
	etnaviv-gpu 38008000.gpu: model: GC520, revision: 5341

This seems to work fine more or less without any changes to the HWDB,
which still might be needed in the future to correct some features,
etc.

[lst]: Added power domains and switched clock assignments to the
       new clock defines used for the composite clocks, instead of
       relying on the backwards compat defines.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>