ARM64: dts: meson-axg: add saradc support

Add the DT info for SAR ADC of the Amlogic's Meson-AXG SoC.

Signed-off-by: Xingyu Chen <xingyu.chen@amlogic.com>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>