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>

arch/arm/Kconfig

@@ -421,9 +421,6 @@ config ARCH_EFM32
 	depends on !MMU
 	select ARCH_REQUIRE_GPIOLIB
 	select ARM_NVIC
-	# CLKSRC_MMIO is wrong here, but needed until a proper fix is merged,
-	# i.e. CLKSRC_EFM32 selecting CLKSRC_MMIO
-	select CLKSRC_MMIO
 	select CLKSRC_OF
 	select COMMON_CLK
 	select CPU_V7M