arm64: dts: meson: g12a: set uart_ao clocks

Now that the AO clock controller is available, make the uarts of the
always-on domain claim the appropriate peripheral clock.

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