clockevents/drivers/Kconfig: Replace USE_OF with OF

USE_OF is used as intermediate Kconfig option by few arch's (ARM, MIPS,
Xtensa); in all these cases it implies setting option OF too. Replace the
only instance of USE_OF in clocksource with OF.

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>