iio: dac: stm32: add power management support

Add support for runtime PM & sleep.
Provide pclk to regmap as registers access doesn't need full power
(e.g. regulator). Always restore HFSEL when resuming. It may get lost
depending on low power level that has been achieved.

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