doc/usage: cmd: documentation for meson/sm command

Added docs for Meson Security Monitor command.

Signed-off-by: Alexey Romanov <avromanov@sberdevices.ru>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/r/20230110105650.54580-4-avromanov@sberdevices.ru


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