iio: adc: stm32: add support for lptimer triggers

STM32 ADC supports hardware triggers like STM32 Low-Power Timer.
For instance, STM32H7 ADC may be triggered by instances 1, 2 or 3.
Add hardware triggered mode so Low-Power Timer Trigger driver can
validate device.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>