arm64: dts: imx8mm: Reduce GPU to nominal speed

When the GPU nodes were added, the GPU_PLL_OUT was configured
for 1000MHz, but this requires the SoC to run in overdrive mode
which requires an elevated voltage operating point.

Since this may run some boards out of spec, the default clock
should be set to 800MHz for nominal operating mode. Boards
that run at the higher voltage can update their clocks
accordingly.

Fixes: 4523be8e

 ("arm64: dts: imx8mm: Add GPU nodes for 2D and 3D core")
Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>