counter: stm32-timer-cnt: add power management support

Add suspend/resume PM sleep ops. When going to low power, enforce the
counter isn't active. Gracefully restore its state upon resume in case
it's been left enabled prior to suspend.

Acked-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>