arm64: dts: meson-axg: add secure monitor

Add the secure monitor device to the axg platform.
With this, we can read the SoC serial number.

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