sandbox: dts: add meson secure monitor node

We need this to test UCLASS_SM.

Signed-off-by: Alexey Romanov <avromanov@salutedevices.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Link: https://lore.kernel.org/r/20230921081346.22157-4-avromanov@salutedevices.com


Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>