iio: counter: Add support for STM32 LPTimer

Add support for STM32 Low-Power Timer, that can be used as counter
or quadrature encoder.

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>