arm64: dts: meson-axg-s400: Add chosen and memory nodes

Add missing chosen and memory nodes.

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