clocksource/drivers/fttmr010: Fix the clock handling

We need to also prepare and enable the clock we are using to get
the right reference count and avoid it being shut off.

Tested-by: Jonas Jensen <jonas.jensen@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>