iio: dac: add support for stm32 DAC

Add support for STMicroelectronics STM32 DAC. It's a 12-bit, voltage
output digital-to-analog converter. It has two output channels, each
with its own converter.
It supports 8 bits or 12bits left/right aligned data format. Only
12bits right-aligned is used here. It has built-in noise or
triangle waveform generator, and supports external triggers for
conversions.
Each channel can be used independently, with separate trigger, then
separate IIO devices are used to handle this. Core driver is intended
to share common resources such as clock, reset, reference voltage and
registers.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>