In the clocksource driver, we didn't explicitly enable the clock. it makes the
clk reference counter wrong. We didn't encounter any hang issue because the
tick's clock input has been open and is shared by some other hardware
components, but if we don't enable those components in kernel, in the stage of
disabling unused clk in kernel boot, Linux tick hangs.

This patch fixes it. it does an explicit prepare and enable to the clock input,
and increases the usage counter of the clk.
Signed-off-by: Zhiwu Song <Zhiwu.Song@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>