Moving some code in common to be able to manage several ADC instances,
or several channels of an instance.

The change involves:
- moving dac_s structure definition to common_object.h
- create TARGET_STM/analogout_api.c and move fully common analog_out
functions in there
- rename analogout_api.c of each target family into analogout_device.c
to keep platform specific code
- update analogout_device.c to rely on obj->handle and obj->channel
- align analogout_init function as much as possible between families in
analogout_device.c files