arm64: dts: allwinner: h6: Add GPU OPP table

Add an Operating Performance Points table for the GPU to
enable Dynamic Voltage & Frequency Scaling on the H6.

The voltage range is set with minimal voltage set to the target
and the maximal voltage set to 1.2V. This allow DVFS framework to
work properly on board with fixed regulator.

Signed-off-by: Clément Péron <peron.clem@gmail.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20220906153034.153321-4-peron.clem@gmail.com