ARM64: dts: meson: enable the saradc node in meson-gx-p23x-q20x.dtsi

meson-gxl-s905d-p230.dts and meson-gxm-q200.dts enable the saradc node
(and configure it's vref-supply "VDDIO_AO18") in their corresponding
.dts file.
Move both (the saradc node as well as the VDDIO_AO18 regulator) to
remove some duplicate code.

As a positive side-effect this enables the saradc also for the P231 (GXL
S905D) and Q201 (GXM S912) development boards which are similar to the
P230/Q200 boards (P231 and Q201 use the internal 100Mbit/s PHY, while
P230 and Q200 have an external RGMII PHY).

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