clocksource: Add Low Power STM32 timers driver

Implement clock event driver using low power STM32 timers.
Low power timer counters running even when CPUs are stopped.
It could be used as clock event broadcaster to wake up CPUs but not like
a clocksource because each it rise an interrupt the counter restart from 0.

Low power timers have a 16 bits counter and a prescaler which allow to
divide the clock per power of 2 to up 128 to target a 32KHz rate.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Pascal Paillet <p.paillet@st.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>