doc: drivers: Include device_config declaration in documentation
doc: drivers: Include device_config declaration in documentation

There are several references to config_info in this documentation, but
no introduction to what it is. Including this declaration makes it
possible to understand the documentation.

Signed-off-by: Sebastian Bøe <edee888afd63e94916688495334700803eba20a0@nordicsemi.no>

explora26/zephyr,finikorg/zephyr,kraj/zephyr,finikorg/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,mbolivar/zephyr,explora26/zephyr,explora26/zephyr,ldts/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,explora26/zephyr,kraj/zephyr,zephyriot/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,mbolivar/zephyr,Vudentz/zephyr,punitvara/zephyr,nashif/zephyr,kraj/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,zephyriot/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyriot/zephyr,galak/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,punitvara/zephyr,ldts/zephyr,punitvara/zephyr,zephyriot/zephyr,galak/zephyr,mbolivar/zephyr,punitvara/zephyr,mbolivar/zephyr,explora26/zephyr,kraj/zephyr