pwm: stm32: Use input prescaler to improve period capture

Using input prescaler, capture unit will trigger DMA once every
configurable /2, /4 or /8 events (rising edge). This helps improve
period (only) capture accuracy at high rates.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>