arm64: dts: amlogic: meson-g12: add the Mali OPP table and use DVFS

Add the OPP table for the Mali Bifrost 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.

Set the GP0_PLL clock to 744MHz (which is the only frequency which
cannot be derived from the FCLK dividers) as the clock driver avoids
setting the parent clock rates so the HIFI PLL clock isn't changed (as
that's reserved for audio).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20200719173213.639540-4-martin.blumenstingl@googlemail.com