clocksource/drivers/fsl_ftm_timer: Fix CLKSRC_MMIO dependency

Select CLKSRC_MMIO when FSL_FTM_TIMER is enabled. Otherwise it fails to
compile on i386 with COMPILE_TEST=y.

"
on i386:
when CLKSRC_MMIO is not enabled:

drivers/built-in.o: In function `ftm_timer_init':
fsl_ftm_timer.c:(.init.text+0x6842): undefined reference to `clocksource_mmio_readl_up'
fsl_ftm_timer.c:(.init.text+0x6855): undefined reference to `clocksource_mmio_init'
"
Reported-by: NRandy Dunlap <rdunlap@infradead.org>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/Kconfig

@@ -257,6 +257,7 @@ config CLKSRC_SAMSUNG_PWM
 config FSL_FTM_TIMER
 	bool "Freescale FlexTimer Module driver" if COMPILE_TEST
 	depends on GENERIC_CLOCKEVENTS
+	select CLKSRC_MMIO
 	help
 	  Support for Freescale FlexTimer Module (FTM) timer.