clocksource: Provide timekeeping for efm32 SoCs

An efm32 features 4 16-bit timers with a 10-bit prescaler. This driver
provides clocksource and clock event device using one timer instance
each.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>