clockevents/driversi/mps2: add MPS2 Timer driver

MPS2 platform has simple 32 bits general purpose countdown timers.

The driver uses the first detected timer as a clocksource and the rest
of the timers as a clockevent

Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>