clocksource: sti: Provide support for the ST LPC Clocksource IP

This IP is shared with Watchdog and RTC functionality.  All 3 of
these devices are mutually exclusive from one another i.e. Only 1
IP can be used at any given time.  We use the device-driver model
combined with a DT 'mode' property to enforce this.

The ST LPC Clocksource IP can be used as the system (tick) timer.

Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>