devicetree: make sure DT_INVALID_NODE docstring appears
devicetree: make sure DT_INVALID_NODE docstring appears

Move the definition to a within a doxygengroup, so its documentation
appears in the reference page.

Signed-off-by: Martí Bolívar <9d90ac4c7bf6a305f6bfd81a23c7859bc883380e@nordicsemi.no>

finikorg/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr