drivers/rtc/rtc-coh901331.c: use clk_prepare/unprepare

Make sure we prepare/unprepare the clock for the COH901331 RTC driver as
is required by the clk API especially if you use common clock.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>