arm64: dts: amlogic: meson-gxm: add the Mali OPP table and use DVFS

Add the OPP table for the Mali-T820 GPU and drop the hardcoded initial
clock configuration. This enables GPU DVFS and thus saves power when the
GPU is not in use while still being able switch to a higher clock on
demand.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20200719173213.639540-3-martin.blumenstingl@googlemail.com