clocksource/drivers/npcm: select TIMER_OF

When this is disabled, we get a link failure:

drivers/clocksource/timer-npcm7xx.o: In function `npcm7xx_timer_init':
timer-npcm7xx.c:(.init.text+0xf): undefined reference to `timer_of_init'

Fixes: 1c00289e ("clocksource/drivers/npcm: Add NPCM7xx timer driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>