iio: adc: stm32-dfsdm: add support for scan mode

In order to support multiple channels in buffer mode, add support for scan
mode. This is precursor patch to ease support of triggered buffer mode.
Currently, only audio uses buffer mode: Regular continuous conversions
with a single channel (per filter).
DFSDM hardware supports scan mode (only) with injected conversions.
Conversions can be launched by software (JSWSTART), trigger or
synchronously with filter 0 (e.g. JSYNC). Continuous conversion mode isn't
available for injected.
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>