arm64: dts: amlogic: meson-gx: add the Mali-450 OPP table and use DVFS

Add the OPP table for the Mali-450 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 MPLL clocks aren't changed (as
these are reserved for audio). The only exception to this is the GXL
S805X package because the 744MHz OPP isn't working correctly there.

While here, make most of meson-gxl-mali re-usable to reduce the amount
of duplicate code between GXBB and GXL. This is more important now as we
don't want to duplicate the GPU OPP table.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20200719173213.639540-2-martin.blumenstingl@googlemail.com