arm64: dts: imx8mp: Add NPU Node

The NPU is based on the Vivante GC8000 and its power-domain
is controlled my pgc_mlmix.  Since the power-domain uses
some of these clocks, setup the clock parent and rates
inside the power-domain, and add the NPU node.

The data sheet states the CLK_ML_AHB should be 300MHz for
nominal, but 800MHz clock will divide down to 266 instead.
Boards which operate in over-drive mode should update the
clocks on their boards accordingly.  When the driver loads,
the NPU numerates as:

 etnaviv-gpu 38500000.npu: model: GC8000, revision: 8002

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>