iio: counter: stm32-lptimer: Add power management support

Add suspend/resume PM sleep ops. When going to low power, disable
active counter. Only active counter should be resumed: don't touch
disabled counter, as it may be used by other LPTimer MFD child driver.

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