dt-bindings: clock: add compatible variant for the Meson-AXG

Update the documentation to support clock driver for the Amlogic's
Meson-AXG SoC.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>