clk: meson: aoclk: refactor common code into dedicated file

We try to refactor the common code into one dedicated file,
while preparing to add new Meson-AXG aoclk driver, this would
help us to better share the code by all aoclk drivers.

Suggested-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>