iio: trigger: Add STM32 LPTimer trigger driver

Add support for LPTIMx_OUT triggers that can be found on some STM32
devices. These triggers can be used then by ADC or DAC.
Typical usage is to configure LPTimer as PWM output (via pwm-stm32-lp)
and have synchronised analog conversions with these triggers.

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