iio: adc: stm32-dfsdm: Add support for stm32mp1

Add support for DFSDM (Digital Filter For Sigma Delta Modulators)
to STM32MP1. This variant is close to STM32H7 DFSDM, it implements
6 filter instances. Registers map is also increased.

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>