clocksource: em_sti: convert to clk_prepare/unprepare

Add calls to clk_prepare and unprepare so that EMMA Mobile EV2 can
migrate to the common clock framework.

Signed-off-by: Shinya Kuribayashi <shinya.kuribayashi.px@renesas.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>