dt-bindings: iio: Add STM32 LPTimer quadrature encoder and counter

Add documentation for STMicroelectronics STM32 Low-Power Timer
quadrature encoder and counter binding.

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