ARM64: dts: meson-axg: remove vddio_ao18 from SoC dtsi

Regulator should not be defined inside the SoC dtsi file.
vddio_ao18 is already defined in the S400 board dts anyway.

Fixes: bb8a2ebd0498 ("ARM64: dts: meson-axg: add saradc support")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>