iio: trigger: stm32-timer: add support for STM32H7

Add support for STM32H7 timer triggers:
- Add new valids_table
- Introduce compatible, with configuration data
- Extend up to 15 timers, available on STM32H7

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