STM32: AnalogOut: do not call HAL_DAC_Start in dac_write ...

dac_write (used by AnalogOut.write) calls HAL_DAC_Start
every time. It is required to call HAL_DAC_Start only once.
HAL_DAC_Start uses internally HAL_Delay(1) making AnalogOut
not suitable for use in high speed application.
This change removes call to HAL_DAC_Start in dac_write and
moves it to analogout_init.

STM32: analog_out: generalize code for multiple instance handling ...

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