clk: meson: add g12a cpu dynamic divider driver

Add a clock driver for the cpu dynamic divider, this divider needs
to have a flag set before setting the divider value then removed
while writing the new value to the register.

This drivers implements this behavior and will be used essentially
on the Amlogic G12A and G12B SoCs for cpu clock trees.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>