arm64: dts: allwinner: Add GPU operating points for H6

The GPU driver needs them to change the clock frequency and regulator
voltage depending on the load.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Clément Péron <peron.clem@gmail.com>

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Feel free to pick up this patch if you are going to keep pushing this
series forward.

Thanks,

Tomeu