ARM: efm32: drop selecting CLKSRC_MMIO

CLKSRC_MMIO is needed on efm32 because the clocksource driver
(CLKSRC_EFM32) uses clocksource_mmio_init() and
clocksource_mmio_readl_up(). Since commit 09ca2757 (clocksource:
time-efm32: Select CLKSRC_MMIO) CLKSRC_EFM32 does the necessary select
itself and ARCH_EFM32 doesn't need to select it explicitly any more.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>