clocksource: sh_cmt: Add clk_prepare/unprepare support

Prepare the clock at probe time, as there is no other appropriate place
in the driver where we're allowed to sleep.

Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>