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: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>