iio: trigger: stm32-timer: add power management support

Add suspend/resume PM sleep ops to stm32-timer-trigger driver.
Register contents may be lost depending on low power modes.
When going to low power, enforce the timer isn't active. Gracefully
restore its state upon resume in case it's been left enabled prior to
suspend.

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