clk: meson: add minimal driver for axg-ao clocks

Add minimal driver AO clocks on meson AXG family. Only ADC related clocks
are supported.

Signed-off-by: Vyacheslav Bocharov <adeep@lexina.in>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lore.kernel.org/r/20220424082159.757622-2-adeep@lexina.in