clocksource: time-efm32: Select CLKSRC_MMIO

The time-efm32 driver uses the clocksource MMIO functions.
Thus it needs to select CLKSRC_MMIO in Kconfig.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>