dt-bindings: iio: adc: stm32-dfsdm: fix types, add missing pinctrl

- Add missing pinctrl description. Support is made optional as dfsdm
  may use internal sources (e.g. via registers)
- Fix typo in IIO STM32 DFSDM filter "MANCH_F" description.
Basically, this should be "falling edge = logic 0", not "1" that applies
to "MANCH_R".
BTW, make the description complete by describing both rising/falling
edges as described in reference manuals.

Fixes: 6c82f947 ("IIO: add DT bindings for stm32 DFSDM filter")

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>