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

drivers/clocksource/Kconfig

@@ -75,6 +75,7 @@ config CLKSRC_DBX500_PRCMU_SCHED_CLOCK
 config CLKSRC_EFM32
 	bool "Clocksource for Energy Micro's EFM32 SoCs" if !ARCH_EFM32
 	depends on OF && ARM && (ARCH_EFM32 || COMPILE_TEST)
+	select CLKSRC_MMIO
 	default ARCH_EFM32
 	help
 	  Support to use the timers of EFM32 SoCs as clock source and clock